static layout: Support field mode

Enable support for field mode, which sets "fieldmode=true" in u-boot
env.

Tested: Verify field mode is enabled via REST API, and verify it is
        still enabled after BMC reboot.

Change-Id: I9ff01a51260d1f08021e0ba7ade16fd48c8bb24e
Signed-off-by: Lei YU <mine260309@gmail.com>
1 file changed