mvebu: image: fix generic-arm64.bootscript mmc selection

Not all versions of ESPRESSObin require SD card, but can
be booted from the internal emmc flash (mmc dev 1) instead.
Add a simple check in the bootscript to see which mmc device
is detected and boot from it using mmcdev variable.

Tested-by: Tomasz Maciej Nowak <[email protected]>
Signed-off-by: Vladimir Vid <[email protected]>

target/linux/mvebu/image/generic-arm64.bootscript

@@ -4,7 +4,13 @@ if test -n "${console}"; then
 	setenv bootargs "${bootargs} ${console}"
 fi
 
-load mmc 0:1 ${fdt_addr} @[email protected]
-load mmc 0:1 ${kernel_addr} Image
+if mmc dev 0; then
+	setenv mmcdev 0
+elif mmc dev 1; then
+	setenv mmcdev 1
+fi
+
+load mmc ${mmcdev}:1 ${fdt_addr} @[email protected]
+load mmc ${mmcdev}:1 ${kernel_addr} Image
 
 booti ${kernel_addr} - ${fdt_addr}