[coreboot] nrv2b cleanup patch

[Myles Watson]

This patch changes do_nrv2b_compress to act like do_lzma_compress.  In other
words, it now allocates and frees its own memory.  This saves lar from having
to know about worst case compression to avoid memory corruption.

Tested by building coreboot with nrv2b as default, then copying to an lzma lar,
then copying back to an nrv2b lar.  No differences.

Myles

Signed-off-by: Myles Watson