uboot-sunxi: use UUID of bootdev and bootpart

Several sunxi devices come with multiple mmc devices. On such devices,
the mmc device order is unpredictable, so using /dev/mmcblk0p2 as root
device doesn't always work, which results in unbootable devices.

For the Banana Pi BPI-R3 in the mediatek target, this has been solved by
defining aliases for the mmc devices in the DTS. Ideally we would do the
same here, but for sunxi-a64 we already use UUID probing, so let's start
with that (5f2ff60 ("uboot-sunxi: a64: allow booting directly from
eMMC")).

Since we're building and including u-boot in each supported device
image, and this method has been proven to work fine for a64, let's just
change the default u-boot env file to do the same.

Fixes: openwrt#10080
Fixes: e6d9f6f ("sunxi: add support for FriendlyARM NanoPi R1")
Co-authored-by: Karl Palsson <karlp@etactica.com>
Signed-off-by: Jan-Niklas Burfeind <git@aiyionpri.me>
[use UUID in default u-boot env, rewrite commit message]
Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>

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

@@ -1,6 +1,8 @@
 setenv fdt_high ffffffff
-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
+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
 setenv uenvcmd run loadkernel \&\& run loaddtb \&\& bootm \$kernel_addr_r - \$fdt_addr_r
 run uenvcmd