cmake: DT: Add support for out-of-tree binding root directories

Just like board's can be placed in out-of-tree BOARD_ROOT's, we now
support DeviceTree sources and bindings being placed in out-of-tree
DTS_ROOT's.

This required for out-of-tree drivers that use DeviceTree.

To implement this we get rid of various user-settable CMake variables
like DTS_APP_BINDINGS, DTS_APP_INCLUDE, and instead have ZEPHYR_BASE,
APPLICATION_SOURCE_DIR and out-of-tree directories conform to using
the same DTS_ROOT concept and directory structure.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>