buildman: Add --allow-missing flag to allow missing blobs

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.

Allow the settings file to control this.

Cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/.azure-pipelines.yml b/.azure-pipelines.yml
index bda7624..665b5d2 100644
--- a/.azure-pipelines.yml
+++ b/.azure-pipelines.yml
@@ -553,7 +553,7 @@
           cat << "EOF" >> build.sh
           if [[ "${BUILDMAN}" != "" ]]; then
               ret=0;
-              tools/buildman/buildman -o /tmp -P -E -W ${BUILDMAN} ${OVERRIDE} || ret=$?;
+              tools/buildman/buildman -o /tmp -PEWM ${BUILDMAN} ${OVERRIDE} || ret=$?;
               if [[ $ret -ne 0 ]]; then
                   tools/buildman/buildman -o /tmp -seP ${BUILDMAN};
                   exit $ret;