Wip/martyn/u boot installer

overlays/installer-mx6qsabrelite/boot.cmd

+echo "Copying U-Boot from SD card to RAM"
+# mmc dev 0
+# mmc read 0x12000000 0x400 0xc0000
+load mmc 0:1 0x12000000 /deb-binaries/usr/lib/u-boot/mx6qsabrelite/u-boot.imx
+echo "Probing SPI-NOR"
+sf probe 0
+echo "Erasing SPI_NOR"
+sf erase 0x0 0xc0000
+echo "Writing U-Boot to SPI-NOR"
+sf write 0x12000000 0x400 0xc0000
+echo "Rebooting"
+reset