buildman: Add --allow-missing-binaries flag to build with BINMAN_ALLOW_MISSING=1

Add a new flag to buildman so that we will in turn pass
BINMAN_ALLOW_MISSING=1 to 'make'. Make use of this flag in CI.

Cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>