Beaglebone: btrfs support, uImage/uinitrd fixes

Added support for btrfs on beaglebone target. (It should still support ext4 as well.)

flash-kernel has a lot of issues running on vfat, so use mkimage instead to produce the uImage and uinitrd for beaglebone. (This is similar to what we do for dreamplug.)

I'll also send a patch for freedom-maker, to add the beaglebone target.