firmware: Add context flag for fastboot policy

Fastboot should be available only in certain conditions i.e.:
 - In developer mode including forced with GBB flag
 - In any mode with Fastboot GBB flag enabled

This commit adds a new context flag which is set based on the above
conditions.

BRANCH=None
BUG=b:484260435
TEST=make runtests # In CrOS-SDK
TEST=Verify that fastboot is unavailable w/o disabling OEM Lock or
setting GBB flags

Change-Id: I70bc431785d1b802231b8a0b363cf8cf0fd0e8a1
Signed-off-by: Jakub Czapiga <czapiga@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/7689794
Reviewed-by: Tomasz Michalec <tmichalec@google.com>