binman: Support an assumed size for missing binaries

binman: Support an assumed size for missing binaries

Binman has a the useful feature of handling missing external blobs
gracefully, including allowing them to be missing, deciding whether the
resulting image is functional or not and faking blobs when this is
necessary for particular tools (e.g. mkimage).

This feature is widely used in CI. One drawback is that if U-Boot grows
too large to fit along with the required blobs, then this is not
discovered until someone does a 'real' build which includes the blobs.

Add a 'assume-size' property to entries to allow Binman to reserve a
given size for missing external blobs.

Signed-off-by: Simon Glass <sjg@chromium.org>