[coreboot] Makefile simplification for v3

[Myles Watson]

These patches are nearly identical.  The only difference is which
directory lar is invoked from.

They do away with lar.tmp because path handling in lar has been fixed.
 In other words, there is no need for copying files around.  Just use
them.

I like the second version better, because it makes it clear where all
the files live by prepending $(obj) in the right places.  I included
both versions because I think it is a matter of taste.

lar -l is identical before and after the patch, and it still boots
with and without payloads in qemu.

Myles

Signed-off-by: Myles Watson