uboot-sunxi: a64: allow booting directly from eMMC

Current boot script uses hardcoded bootdevice, which allows booting from
SD card only, so this patch allows booting directly from eMMC as well.
While at it, replace fixed root device with more flexible UUID based
probing, so from now on probing order of MMC device doesn't matter.

Signed-off-by: Petr Štetiar <ynezz@true.cz>

package/boot/uboot-sunxi/uEnv-a64.txt

@@ -1,5 +1,7 @@
-setenv loadkernel fatload mmc 0 \$kernel_addr_r uImage
-setenv loaddtb fatload mmc 0 \$fdt_addr_r dtb
-setenv bootargs console=ttyS0,115200 earlyprintk root=/dev/mmcblk0p2 rootwait earlycon=uart,mmio32,0x01c28000
+setenv mmc_rootpart 2
+part uuid mmc ${mmc_bootdev}:${mmc_rootpart} uuid
+setenv loadkernel fatload mmc \$mmc_bootdev \$kernel_addr_r uImage
+setenv loaddtb fatload mmc \$mmc_bootdev \$fdt_addr_r dtb
+setenv bootargs console=ttyS0,115200 earlyprintk root=PARTUUID=${uuid} rootwait earlycon=uart,mmio32,0x01c28000
 setenv uenvcmd run loadkernel \&\& run loaddtb \&\& booti \$kernel_addr_r - \$fdt_addr_r
 run uenvcmd