g: signer: use strings instead of numbers for Board ID fields
g: signer: use strings instead of numbers for Board ID fields

Recent signer improvements allow to include hex values into the
manifest, this also makes it easier to process values with bid D31
set, let's use strings instead of ints.

BRANCH=none
BUG=none
TEST=verified signing of a prod release

Change-Id: Ia05198991bfdd8dfd831545a9edf70f1e06f24f4
Signed-off-by: Vadim Bendebury <5515d6d2d0829cbe0dd0dcf2094aaded06d58514@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/604217
Reviewed-by: Mary Ruthven <81380f3403fed74252e92df3ed012a1819ec6296@chromium.org>

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