cache coreboot builds in elf/ again

This was a problem when I did it before, because individual
target builds weren't automatically re-compiled when needed.

The recent design improvements in lbmk enable this to be
done again.

Cached images in elf/ have no payloads, so they are a liability,
therefore they are padded by one byte to prevent flashing. This
solves the problem that the previous caching had.

With this change, modifying payloads can be tested without
needing to re-build coreboot each time.

The cached coreboot builds are also automatically re-built when
needed, which is another improvement this time, compared to
the last time coreboot builds were cached in this manner.

Signed-off-by: Leah Rowe <leah@libreboot.org>

config/data/coreboot/build.list

@@ -0,0 +1 @@
+build/coreboot.rom

config/data/coreboot/mkhelper.cfg

@@ -12,5 +12,5 @@ eval `setvars "n" $pv`
 eval `setvars "" $v`
 
 premake="corebootpremake"
-mkhelper="mkcorebootbin"
-postmake="mkcoreboottar"
+mkhelper="coreboot_pad_one_byte"
+postmake="mkcorebootbin"