gbb_flags_common.sh: Add GBB_FLAG_ENABLE_UDC to help text
gbb_flags_common.sh: Add GBB_FLAG_ENABLE_UDC to help text

BUG=b:78577893
BRANCH=None
TEST=None

Change-Id: I1905f53aadb0ae882dc4cf9f4fd214b4ccdfc440
Signed-off-by: Furquan Shaikh <6be5c96c464778f89679b1062a2e0a6af93d2aa5@google.com>
Reviewed-on: https://chromium-review.googlesource.com/1029429
Commit-Ready: Furquan Shaikh <6be5c96c464778f89679b1062a2e0a6af93d2aa5@chromium.org>
Tested-by: Furquan Shaikh <6be5c96c464778f89679b1062a2e0a6af93d2aa5@chromium.org>
Reviewed-by: Duncan Laurie <70662b43bc1a20b57ded72a2b08be88dd0da4841@google.com>

coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot