[coreboot] v3 build process and lar flags

[Myles Watson]

It seems like the build would be cleaner if we had two lists of files to add
to the lar and added them in two steps.

lar -c coreboot.rom $(NOCOMPRESS_FILES) -s $(ROM_SIZE) ...
lar -a coreboot.rom $(COMPRESS_FLAG) $(COMPRESSIBLE_FILES)

The unfortunate thing with the nocompress:file:lar_path style is that it
makes it difficult to add support for other flags, unless you specify a flag
ordering.  For example imagine if you wanted to add a flag specifying to add
the file as close to the end as possible, etc.

The reason I was interested is I still think the cleanest way is to allow
repacking of lar files (i.e., no intermediate format), but I don't see a
good way to add that option with the current usage model for lar.

Myles