warp: Use PARTUUID to specify the rootfs location

warp can run different kernel versions, such as NXP 4.1 or
mainline.

Currently the rootfs location is passed via mmcblk number and the
problem with this approach is that the mmcblk number for the eMMC
changes depending on the kernel version.

In order to avoid such issue, use UUID method to specify the rootfs
location.

Succesfully tested booting a NXP 4.1 and also a mainline 4.12 kernel.
Signed-off-by: NFabio Estevam <fabio.estevam@nxp.com>
Acked-by: NOtavio Salvador <otavio@ossystems.com.br>

configs/warp_defconfig

@@ -10,6 +10,7 @@ CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_MMC=y
+CONFIG_CMD_PART=y
 CONFIG_CMD_I2C=y
 CONFIG_CMD_USB=y
 CONFIG_CMD_DFU=y

include/configs/warp.h

@@ -95,10 +95,10 @@
 	"ip_dyn=yes\0" \
 	"mmcdev=0\0" \
 	"mmcpart=1\0" \
-	"mmcroot=/dev/mmcblk0p2 rootwait rw\0" \
+	"finduuid=part uuid mmc 0:2 uuid\0" \
 	"dfu_alt_info=boot raw 0x2 0x400 mmcpart 1\0" \
 	"mmcargs=setenv bootargs console=${console},${baudrate} " \
-		"root=${mmcroot}\0" \
+		"root=PARTUUID=${uuid} rootwait rw\0" \
 	"loadbootscript=" \
 		"fatload mmc ${mmcdev}:${mmcpart} ${loadaddr} ${script};\0" \
 	"bootscript=echo Running bootscript from mmc ...; " \
@@ -106,6 +106,7 @@
 	"loadimage=fatload mmc ${mmcdev}:${mmcpart} ${loadaddr} ${image}\0" \
 	"loadfdt=fatload mmc ${mmcdev}:${mmcpart} ${fdt_addr} ${fdt_file}\0" \
 	"mmcboot=echo Booting from mmc ...; " \
+		"run finduuid; " \
 		"run mmcargs; " \
 		"if test ${boot_fdt} = yes || test ${boot_fdt} = try; then " \
 			"if run loadfdt; then " \