vboot: relocate GBB flag definitions to 2gbb_flags.h

vb2_gbb_header should NOT be in the public API, but GBB flag
definitions themselves SHOULD be.

BUG=b:124141368, chromium:956474
TEST=make clean && make runtests
BRANCH=none

Change-Id: I9b9cb1bc67c31c3a29b7a237d90a11aba55f131e
Signed-off-by: Joel Kitching <kitching@google.com>
Reviewed-on: https://chromium-review.googlesource.com/1583821
Commit-Ready: Joel Kitching <kitching@chromium.org>
Tested-by: Joel Kitching <kitching@chromium.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>