cr50: Only allow SN hash to be written if board ID is not set.
cr50: Only allow SN hash to be written if board ID is not set.

This should restrict SN hash such that is can only be written at
factory (before board id), with the exception of some edge cases
where devices that have left factory do not have a board id set.

BUG=b:111195266
TEST=tested locally on soraka
BRANCH=none

Change-Id: I2ae39e2db4b1a01ec5ec9855634357434f01020b
Signed-off-by: Louis Collard <591722de08e49b258a33cc28248aa0c58c1a6929@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1237696
Reviewed-by: Andrey Pronin <53bbebe6910b11f7aa2d1daaa466658eb4cba5f2@chromium.org>

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