warp7: Set u-boot serial# based on OTP value

u-boot has a standard "serial#" environment variable that is suitable
for storing the iSerial number we will supply via the USB device
descriptor. serial# is automatically picked up by the disk subsystem in
u-boot - thus providing a handy unique identifier in /dev/disk/by-id as
detailed below.

Storing the hardware serial identifier in serial# means we can change the
serial# if we want before USB enumeration - thus making iSerial automatic
via OTP but overridable if necessary.

This patch reads the defined OTP fuse and sets environment variable
"serial#" to the value read.

With this patch in place the USB mass storage device will appear in
/dev/disk/by-id with a unique name based on the OTP value. For example

/dev/disk/by-id/usb-Linux_UMS_disk_0_WaRP7-0xf42400d3000001d4-0:0

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Rui Miguel Silva <rui.silva@linaro.org>
Cc: Ryan Harkin <ryan.harkin@linaro.org>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
diff --git a/include/configs/warp7.h b/include/configs/warp7.h
index fe96988..0c3b605 100644
--- a/include/configs/warp7.h
+++ b/include/configs/warp7.h
@@ -24,6 +24,9 @@
 #define CONFIG_SYS_FSL_ESDHC_HAS_DDR_MODE
 #define CONFIG_SYS_MMC_IMG_LOAD_PART	1
 
+/* Switch on SERIAL_TAG */
+#define CONFIG_SERIAL_TAG
+
 #define CONFIG_DFU_ENV_SETTINGS \
 	"dfu_alt_info=boot raw 0x2 0x400 mmcpart 1\0" \