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/tools/buildman/builderthread.py b/tools/buildman/builderthread.py
index 065d836..680efae 100644
--- a/tools/buildman/builderthread.py
+++ b/tools/buildman/builderthread.py
@@ -253,6 +253,8 @@
                     args.extend(['-j', str(self.builder.num_jobs)])
                 if self.builder.warnings_as_errors:
                     args.append('KCFLAGS=-Werror')
+                if self.builder.allow_missing:
+                    args.append('BINMAN_ALLOW_MISSING=1')
                 config_args = ['%s_defconfig' % brd.target]
                 config_out = ''
                 args.extend(self.builder.toolchains.GetMakeArguments(brd))