binman: Make fake blobs zero-sized by default

On x86 devices having even a small amount of data can cause an overlap
between regions. For example, bayleybay complains when the intel-vga
region overlaps with u-boot-ucode:

   ImagePos    Offset      Size  Name
   <none>    00000000  00800000  main-section
   <none>     ff800000  00000080  intel-descriptor
   <none>     ff800400  00000080  intel-me
   <none>     fff00000  00098f24  u-boot-with-ucode-ptr
   <none>     fff98f24  00001aa0  u-boot-dtb-with-ucode
   <none>     fff9a9d0  0002a000  u-boot-ucode
   <none>     fffb0000  00000080  intel-vga
   ...

It is safer to use an empty file in most cases. Add an option to set the
size for those uses that need it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>