[coreboot] patch: add zero fill command option to lar

[ron minnich]

You can now say:
lar z build/bios.bin

and get a zero-filled lar entry. The "zeros" are actually 0xff, which
is flash-friendly and makes flashrom fast.

Parts of stream.c could use the new functions, in particular hlen, but
that patch is for later.

This change dramatically speeds up booting on the alix 1c.

ron