device-tree.bb: Rework pre-processor and dtc flags
device-tree.bb: Rework pre-processor and dtc flags

Add 'DEVICETREE_PP_FLAGS' for handling of flags for the pre-processor
step. Also merge the building of the include path args into the
variables with inline python.

Use BUILD_CPP instead of CPP to avoid dependence on the target
toolchain.

Signed-off-by: Nathan Rossi <2e8aa918660411855c6d44d5bb2da677aa033255@nathanrossi.com>

Xilinx/meta-xilinx,Xilinx/meta-xilinx,Xilinx/meta-xilinx,Xilinx/meta-xilinx,Xilinx/meta-xilinx,nathanrossi/meta-xilinx,nathanrossi/meta-xilinx,Xilinx/meta-xilinx,nathanrossi/meta-xilinx,nathanrossi/meta-xilinx