vboot1: Condition default legacy boot on dev_boot_legacy

This patch fixes what I think is an inconsistency in the existing legacy
boot behavior: when the GBB flag that defaults to legacy boot is set,
running out the 30 second timer would still boot legacy mode even if
dev_boot_legacy is not actually set (whereas pressing CTRL+L in the
same configuration would beep and refuse).

This patch makes both legacy boot trgiggers check the same condition
before boot. This does not restrict functionality since anyone who sets
the DEFAULT_DEV_BOOT_LEGACY GBB flag could simply set
FORCE_DEV_BOOT_LEGACY at the same time. It does however open up an
interesting new use case of using NVRAM to change back-and-forth between
legacy and normal developer mode (after GBB flags are changed once and
write-protection is enabled again).

If this is updated in the field it might lock existing devices out of
legacy mode... however, since by far the most common GBB flag
combination recommended on the internet seems to be 0x489 (including
FORCE_DEV_BOOT_LEGACY), I doubt this would be a problem in practice.

TEST=Booted with GBB flags 0x4b9 and 0x439, observed difference.

Change-Id: If6a6d99ab2cf116db2237fdc3df97fc22a68251c
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/270182
Reviewed-by: Duncan Laurie <dlaurie@chromium.org>
2 files changed