Likely due the merge of CL https://chrome-internal-review.googlesource.com/#/c/375491/, currently reef-uni's build is broken with log like,

coreboot-0.0.1-r2131:     CREATE GBB (without BMPFV)
coreboot-0.0.1-r2131: successfully created new GBB to: build-reef/gbb.stub.tmp
coreboot-0.0.1-r2131:     SETUP GBB
coreboot-0.0.1-r2131:  - import root_key from 3rdparty/vboot/tests/devkeys/root_key.vbpubk: success
coreboot-0.0.1-r2131:  - import recovery_key from 3rdparty/vboot/tests/devkeys/recovery_key.vbpubk: success
coreboot-0.0.1-r2131: successfully saved new image to: build-reef/gbb.region.tmp
coreboot-0.0.1-r2131:     WRITE GBB
coreboot-0.0.1-r2131: Image written successfully to build-reef/cbfs/fallback/ifwi.bin.tmp.
coreboot-0.0.1-r2131: Image does not contain sub-partition OBBP(6).
coreboot-0.0.1-r2131: Sub-partition IBBP(4) entry IBBL replaced from file build-reef/cbfs/fallback/bootblock.bin.
coreboot-0.0.1-r2131: Image written successfully to build-reef/cbfs/fallback/ifwi.bin.tmp.
coreboot-0.0.1-r2131: W: Written area will abut bottom of target region: any unused space will keep its current contents
coreboot-0.0.1-r2131: W: Written area will abut bottom of target region: any unused space will keep its current contents
coreboot-0.0.1-r2131: W: Written area will abut bottom of target region: any unused space will keep its current contents
coreboot-0.0.1-r2131: /build/reef-uni/firmware/reef/ps8751/ps8751_a3.bin: No such file or directory
coreboot-0.0.1-r2131: E: Could not load file '/build/reef-uni/firmware/reef/ps8751/ps8751_a3.bin'.
coreboot-0.0.1-r2131: E: Failed while operating on 'FW_MAIN_A' region!
coreboot-0.0.1-r2131: E: The image will be left unmodified.
coreboot-0.0.1-r2131:  * ERROR: sys-boot/coreboot-0.0.1-r2131::chromiumos failed (compile phase):
coreboot-0.0.1-r2131:  *   (no error message)
coreboot-0.0.1-r2131:  * 
coreboot-0.0.1-r2131:  * Call stack:
coreboot-0.0.1-r2131:  *     ebuild.sh, line   93:  Called src_compile
coreboot-0.0.1-r2131:  *   environment, line 3613:  Called make_coreboot 'build-reef' '.config-reef' 'reef'
coreboot-0.0.1-r2131:  *   environment, line 2707:  Called add_fw_blob 'build-reef/coreboot.rom' 'ps8751_a3.bin' '/build/reef-uni/firmware/reef/ps8751/ps8751_a3.bin'
coreboot-0.0.1-r2131:  *   environment, line  466:  Called die
coreboot-0.0.1-r2131:  * The specific snippet of code:
coreboot-0.0.1-r2131:  *       cbfstool "${rom}" add -r FW_MAIN_A,FW_MAIN_B -t raw -c lzma -f "${blob}" -n "${cbname}" || die;
coreboot-0.0.1-r2131:  * 
coreboot-0.0.1-r2131:  * If you need support, post the output of `emerge --info '=sys-boot/coreboot-0.0.1-r2131::chromiumos'`,
coreboot-0.0.1-r2131:  * the complete build log and the output of `emerge -pqv '=sys-boot/coreboot-0.0.1-r2131::chromiumos'`.
coreboot-0.0.1-r2131:  * The complete build log is located at '/build/reef-uni/tmp/portage/logs/sys-boot:coreboot-0.0.1-r2131:20170619-155234.log'.
coreboot-0.0.1-r2131:  * For convenience, a symlink to the build log is located at '/build/reef-uni/tmp/portage/sys-boot/coreboot-0.0.1-r2131/temp/build.log'.
coreboot-0.0.1-r2131:  * The ebuild environment file is located at '/build/reef-uni/tmp/portage/sys-boot/coreboot-0.0.1-r2131/temp/environment'.
coreboot-0.0.1-r2131:  * Working directory: '/build/reef-uni/tmp/portage/sys-boot/coreboot-0.0.1-r2131/work/coreboot-0.0.1'
coreboot-0.0.1-r2131:  * S: '/build/reef-uni/tmp/portage/sys-boot/coreboot-0.0.1-r2131/work/coreboot-0.0.1'
coreboot-0.0.1-r2131: >>> Failed to emerge sys-boot/coreboot-0.0.1-r2131 for /build/reef-uni/, Log file:
coreboot-0.0.1-r2131: >>>  '/build/reef-uni/tmp/portage/logs/sys-boot:coreboot-0.0.1-r2131:20170619-155234.log'
coreboot-0.0.1-r2131: 
coreboot-0.0.1-r2131:  * Messages for package sys-boot/coreboot-0.0.1-r2131 merged to /build/reef-uni/:
coreboot-0.0.1-r2131: 
coreboot-0.0.1-r2131:  * ERROR: sys-boot/coreboot-0.0.1-r2131::chromiumos failed (compile phase):
coreboot-0.0.1-r2131:  *   (no error message)
coreboot-0.0.1-r2131:  * 
coreboot-0.0.1-r2131:  * Call stack:
coreboot-0.0.1-r2131:  *     ebuild.sh, line   93:  Called src_compile
coreboot-0.0.1-r2131:  *   environment, line 3613:  Called make_coreboot 'build-reef' '.config-reef' 'reef'
coreboot-0.0.1-r2131:  *   environment, line 2707:  Called add_fw_blob 'build-reef/coreboot.rom' 'ps8751_a3.bin' '/build/reef-uni/firmware/reef/ps8751/ps8751_a3.bin'
coreboot-0.0.1-r2131:  *   environment, line  466:  Called die
coreboot-0.0.1-r2131:  * The specific snippet of code:
coreboot-0.0.1-r2131:  *       cbfstool "${rom}" add -r FW_MAIN_A,FW_MAIN_B -t raw -c lzma -f "${blob}" -n "${cbname}" || die;
coreboot-0.0.1-r2131:  * 
coreboot-0.0.1-r2131:  * If you need support, post the output of `emerge --info '=sys-boot/coreboot-0.0.1-r2131::chromiumos'`,
coreboot-0.0.1-r2131:  * the complete build log and the output of `emerge -pqv '=sys-boot/coreboot-0.0.1-r2131::chromiumos'`.
coreboot-0.0.1-r2131:  * The complete build log is located at '/build/reef-uni/tmp/portage/logs/sys-boot:coreboot-0.0.1-r2131:20170619-155234.log'.
coreboot-0.0.1-r2131:  * For convenience, a symlink to the build log is located at '/build/reef-uni/tmp/portage/sys-boot/coreboot-0.0.1-r2131/temp/build.log'.
coreboot-0.0.1-r2131:  * The ebuild environment file is located at '/build/reef-uni/tmp/portage/sys-boot/coreboot-0.0.1-r2131/temp/environment'.
coreboot-0.0.1-r2131:  * Working directory: '/build/reef-uni/tmp/portage/sys-boot/coreboot-0.0.1-r2131/work/coreboot-0.0.1'
coreboot-0.0.1-r2131:  * S: '/build/reef-uni/tmp/portage/sys-boot/coreboot-0.0.1-r2131/work/coreboot-0.0.1'