Support booting beaglebone from external rootfs

src/freedombone-image-hardware-setup

@@ -40,6 +40,10 @@ enable_serial_console() {
 
 beaglebone_setup_boot() {
     beaglebone_variant="$1"
+    beaglebone_root_device='mmcblk0p2'
+    if [ "$2" ]; then
+        beaglebone_root_device="$2"
+    fi
 
     # Setup uEnv.txt
     if grep -q btrfs /etc/fstab ; then
@@ -60,7 +64,7 @@ beaglebone_setup_boot() {
     # uEnv.txt for Beaglebone
     # based on https://github.com/beagleboard/image-builder/blob/master/target/boot/beagleboard.org.txt
     cat >> /boot/uEnv.txt <<EOF
-mmcroot=/dev/mmcblk0p2 ro
+mmcroot /dev/${beaglebone_root_device} ro
 mmcrootfstype=$fstype rootwait fixrtc
 mmcrootflags=subvol=@
 
@@ -308,19 +312,19 @@ EOF
 
 case "$MACHINE" in
     beagleboneblack)
-        beaglebone_setup_boot am335x-boneblack
+        beaglebone_setup_boot am335x-boneblack "$EXTERNAL_DRIVE"
         beaglebone_flash "TI AM335x BeagleBone Black"
         beaglebone_repack_kernel am335x-boneblack
         enable_serial_console ttyO0
         ;;
     beaglebonegreen)
-        beaglebone_setup_boot am335x-bonegreen
+        beaglebone_setup_boot am335x-bonegreen "$EXTERNAL_DRIVE"
         beaglebone_flash "TI AM335x BeagleBone Green"
         beaglebone_repack_kernel am335x-bonegreen
         enable_serial_console ttyO0
         ;;
     beaglex15)
-        beaglebone_setup_boot am57xx-beagle-x15
+        beaglebone_setup_boot am57xx-beagle-x15 "$EXTERNAL_DRIVE"
         beaglebone_flash "TI AM5728 BeagleBoard-X15"
         beaglebone_repack_kernel am57xx-beagle-x15
         enable_serial_console ttyO0