x86, mm: Let "memmap=" take more entries one time

Current "memmap=" only can take one entry every time.
when we have more entries, we have to use memmap= for each of them.

For pxe booting, we have command line length limitation, those extra
"memmap=" would waste too much space.

This patch make memmap= could take several entries one time,
and those entries will be split with ','

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Link: http://lkml.kernel.org/r/1353123563-3103-47-git-send-email-yinghai@kernel.org
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>