uboot-zynq: fix boot process on MMC

A regression was introduced when upgrading to 2019.07, whereas
the bootloader did not proceed to load the FIT image. A fix
was reported but not upstreamed here [1]. Patch the uEnv file
to bring back these boards to an operational state.

[1] https://forum.openwrt.org/t/bugreport-regression-in-zynq-cpu-platform-not-bootiing-any-more-zedboard-zybo/182609/7

Fixes: https://git.openwrt.org/?p=openwrt/openwrt.git;a=commit;h=5ca243153b110ceddffecb70ba8a8cd0e33c8f0b
Signed-off-by: Zoltan HERPAI <wigyori@uid0.hu>
(cherry picked from commit fa55634872)

package/boot/uboot-zynq/files/uEnv-default.txt

@@ -1 +1,2 @@
 bootargs=console=ttyPS0,115200n8 root=/dev/mmcblk0p2 rootwait earlyprintk
+bootcmd=run sdboot