warp7: select uuid partition based on rootpart

Assigning the UUID discovery path to a tweakable environment variable means
that later steps in the boot process - particularly a boot script can
change the target root partition of a particular Linux boot.

Retargeting the rootfs is an important feature when doing ping/pong
upgrades allowing a boot script to select ping or pong as necessary without
reprogramming the bootloader.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Tested-by: Breno Lima <breno.lima@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>

include/configs/warp7.h

@@ -45,7 +45,8 @@
 	"ip_dyn=yes\0" \
 	"mmcdev="__stringify(CONFIG_SYS_MMC_ENV_DEV)"\0" \
 	"mmcpart=" __stringify(CONFIG_SYS_MMC_IMG_LOAD_PART) "\0" \
-	"finduuid=part uuid mmc 0:2 uuid\0" \
+	"rootpart=" __stringify(CONFIG_WARP7_ROOT_PART) "\0" \
+	"finduuid=part uuid mmc 0:${rootpart} uuid\0" \
 	"mmcargs=setenv bootargs console=${console},${baudrate} " \
 		"root=PARTUUID=${uuid} rootwait rw\0" \
 	"loadbootscript=" \