Refactor device tree

It's quite a mess right now - this commit makes it a lot cleaner.

Included changes:
- Introduce the DEVICE_FOLDER variable to make paths more readable
- Reorganize makefiles
- Remove obsolete recovery.fstab file

Change-Id: I0fe2d9994afe44207b267e60e237c9c997aa6e72

Conflicts:
	BoardConfig.mk
	device.mk