flash_ec: update zephyr support to support flat projects
flash_ec: update zephyr support to support flat projects

Identify valid Zephyr projects by finding the zmake.yaml file. This
correctly finds Zephyr projects, regardless of the directory layout
under zephyr/projects.

BUG=none
BRANCH=none
TEST="./util/flash_ec --board guybrush --dry_run --zephyr" identifies
the EC binary as "build/zephyr/projects/guybrush/output/zephyr.bin"
TEST="./util/flash_ec --board volteer --dry_run --zephyr" identifies the
EC binary as "build/zephyr/projects/volteer/volteer/output/zephyr.bin"

Signed-off-by: Keith Short <f6d578f97033c25f1206cb57d52f358974c82954@chromium.org>
Change-Id: Ib43aa718f41e4c73f2ad574f7a260f7a94c23dcf
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3212779
Reviewed-by: Jack Rosenthal <d3f605bef1867f59845d4ce6e4f83b8dc9e4e0ae@chromium.org>
Reviewed-by: Diana Z <cb2016afc7183e5356c690dc63b4ba817470501a@chromium.org>
Commit-Queue: Diana Z <cb2016afc7183e5356c690dc63b4ba817470501a@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec