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

Change-Id: Ie609544a460fc991e66e8b95e8d7a3ed5e845f7b
Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/207427
Reviewed-by: Trevor Bourget <tbourget@codeaurora.org>
Reviewed-by: David Hendricks <dhendrix@chromium.org>

src/mainboard/google/storm/Kconfig

@@ -27,16 +27,22 @@ config BOARD_SPECIFIC_OPTIONS
 	select SPI_FLASH
 	select SPI_FLASH_SPANSION
 
+config BOARD_VARIANT_AP148
+	bool "pick this to build an image for ap148"
+	default n
+
 config MAINBOARD_DIR
 	string
 	default google/storm
 
 config MAINBOARD_PART_NUMBER
 	string
+	default "AP148" if BOARD_VARIANT_AP148
 	default "Storm"
 
 config DRAM_SIZE_MB
 	int
+	default 512 if BOARD_VARIANT_AP148
 	default 1024
 
 config DRAM_DMA_START

src/mainboard/google/storm/mainboard.c

@@ -37,8 +37,10 @@
 
 static void setup_usb(void)
 {
+#if !CONFIG_BOARD_VARIANT_AP148
 	gpio_tlmm_config_set(USB_ENABLE_GPIO, FUNC_SEL_GPIO,
 			     GPIO_PULL_UP, GPIO_10MA, GPIO_ENABLE);
+#endif
 	usb_clock_config();
 
 	setup_usb_host1();