omap: support booting off different mmc devices

Tested with installing to/sysupgrading from sd and emmc on boneblack.

Signed-off-by: Andre Heider <a.heider@gmail.com>

package/boot/uboot-omap/files/boot.scr.txt

@@ -1,5 +1,9 @@
-setenv bootargs console=${console} root=/dev/mmcblk0p2 rootwait
+if test -z "${devnum}"; then
+	setenv devnum 0
+fi
 
-load mmc 0:1 ${loadaddr} /zImage \
-&& load mmc 0:1 ${fdtaddr} /dtbs/${fdtfile} \
+setenv bootargs console=${console} root=/dev/mmcblk${devnum}p2 rootwait
+
+load mmc ${devnum}:1 ${loadaddr} /zImage \
+&& load mmc ${devnum}:1 ${fdtaddr} /dtbs/${fdtfile} \
 && bootz ${loadaddr} - ${fdtaddr}