It is useful to be able to distinguish between ordinary blobs such as
u-boot.bin and external blobs that cannot be build by the U-Boot build
system. If the external blobs are not available for some reason, then we
know that a value image cannot be built.

Introduce a new 'blob-ext' entry type for that.
Signed-off-by: NSimon Glass <sjg@chromium.org>
Reviewed-by: NBin Meng <bmeng.cn@gmail.com>