[coreboot] [PATCH]romfs: avoid build errors for non-romfs targets, and support lzma compression for romfs targets

[Patrick Georgi]

Hi,

I thought that romfs infrastructure is done now, but there were some 
issues (see buildbot).
The romfs image was always built, and sometimes broke (because of the 
different image layouts) for buildrom images. After the patch, these 
issues are avoided by not adding payloads to the romfs image (they 
wouldn't be read anyway). Both workarounds (in buildrom code for romfs 
and vice-versa) aren't very pretty, but that's what our buildsystem 
requires.
As I had to create a "communication channel" (via the romfs-support 
files), I took the chance to also use it for compression information, so 
if you configure lzma support, you'll get lzma compressed payloads in romfs.

Signed-off-by: Patrick Georgi

Patrick