probably good to provide reproducer instructions.

I don't want anyone to work on it till Chrome build + graphics sandbox issues are resolved since currently it is a bit of work to get a working image.

Repro instructions are fairly standard:
Use prebuilt images from kevin64-release builder (after Chrome build issues are resolved)
Or build image locally:
$ ./build_packages --board=kevin64 (will build chrome from source so will take a while)
$ ./build_image --board=kevin64 --noenable_rootfs_verification test


# flash the image to a kevin board (usb/locally).
$ cros flash ....

Try running the commands:
# enable_dev_usb_boot 
ERROR: Failed to enable USB boot.

# crossystem dev_boot_usb
waitpid() or mosys error

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform/mosys/+/2bcf2e064e4e680c2270e739f102a0453f1886b8

commit 2bcf2e064e4e680c2270e739f102a0453f1886b8
Author: Adam Kallai <kadam@inf.u-szeged.hu>
Date: Tue Sep 18 13:42:14 2018

mosys: allows some syscalls for arm64

crossystem cannot obtain system specific information or set
any property because of mosys error (i.e. enable dev_boot_usb).

BUG= chromium:882987 
BUG=chromium:878565
TEST=checks with minijail0 and run crossystem

Change-Id: I06632486f2b68b241c87860a2a317450df8954cc
Reviewed-on: https://chromium-review.googlesource.com/1228134
Commit-Ready: Adam Kallai <kadam@inf.u-szeged.hu>
Tested-by: Adam Kallai <kadam@inf.u-szeged.hu>
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>

[modify] https://crrev.com/2bcf2e064e4e680c2270e739f102a0453f1886b8/seccomp/mosys-seccomp-arm64.policy