storm: Provide ability to build ap148 variant

storm: Provide ability to build ap148 variant

With BOARD_VARIANT_AP148 configuration option enabled the image will
be built for 512MB DRAM instead of 1024MB and the
mainboard_part_number field in the lb_mainboard entry will be set to
"AP148" instead of "Storm".

BUG=chrome-os-partner:30440
TEST=manual
   . built and booted both AP148 and proto0 all the way to reading the
     kernel
   . verified that the config file includes correct part number and
     memory size
   . verified proper machine IDs reportted when starting the kernel

Original-Change-Id: Ie609544a460fc991e66e8b95e8d7a3ed5e845f7b
Original-Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/207427
Original-Reviewed-by: Trevor Bourget <tbourget@codeaurora.org>
Original-Reviewed-by: David Hendricks <dhendrix@chromium.org>
(cherry picked from commit a80ab00f27eef9e3aa2f761659d6945d6fce2ef6)
Signed-off-by: Marc Jones <marc.jones@se-eng.com>
Change-Id: I477e672dc4f48fa9c9893bf0759704501ea07b1a
Reviewed-on: http://review.coreboot.org/8590
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: David Hendricks <dhendrix@chromium.org>