When building BOARD=cr50, we get these warnings:

# make BOARD=cr50 -j
  LD      RW/ec.RW.elf
  LD      RW/ec.RW_B.elf
/usr/x86_64-pc-linux-gnu/arm-none-eabi/binutils-bin/2.27.0/ld.bfd.real: warning: section `.bss' type changed to PROGBITS
/usr/x86_64-pc-linux-gnu/arm-none-eabi/binutils-bin/2.27.0/ld.bfd.real: warning: section `.bss' type changed to PROGBITS
  NM      RW/ec.RW.smap
  NM      RW/ec.RW_B.smap
  OBJCOPY RW/ec.RW.hex
  RW_SIGN RW/ec.RW.flat
  RW_SIGN RW/ec.RW_B.flat
  CAT     ec.obj
  OBJCOPY ec.bin
  COPY_RW ec.bin
  MV      ec.bin
  OBJCOPY ec.hex

I did a bisect with this:
git bisect run sh -c "rm -rf build/cr50; if make BOARD=cr50 -j 2>&1 | grep PROGBITS; then exit 1; fi"

And the first bad CL is:

6b516eda69f92cbc9f06b33cdf2c34cf1114ca16 is the first bad commit
commit 6b516eda69f92cbc9f06b33cdf2c34cf1114ca16
Author: Vadim Bendebury <vbendeb@chromium.org>
Date:   Fri Oct 20 10:30:02 2017 -0700

    tpm2: add nvmem data to the right segment
    
    Modules in board/cr50/tpm2 are part of the TPM task, and their .bss
    segment needs to be wiped out along with other TPM modules' when TPM
    is reset.
    
    The only module from this directory which has static variables is
    NVMem.c. This patch groups the variables into a structure, and makes
    sure that the structure is stored in the part of .bss which is zeroed
    when TPM is reset.
    
    BRANCH=none
    BUG=none
    TEST=verified that TPM wipeout happening during RMA reset is
         successful.
    
    Change-Id: I5f80dc5e4f5f7cdce2d1a1ed59e0fbac354c5bf6
    Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
    Reviewed-on: https://chromium-review.googlesource.com/733814
    Reviewed-by: Randall Spangler <rspangler@chromium.org>
    Reviewed-by: Scott Collyer <scollyer@chromium.org>

Not sure if we should be concerned.