powerpc/nvram: Add compression to fit more oops output into NVRAM

Capture more than twice as much text from the printk buffer, and
compress it to fit it in the lnx,oops-log NVRAM partition.  You
can view the compressed text using the new (as of July 20) --unzip
option of the nvram command in the powerpc-utils package.

[BenH: Added select of ZLIB_DEFLATE]

Signed-off-by: Jim Keniston <jkenisto@us.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>