devicetree: generate extern's for devicetree struct devices

Generate a header (device_extern.h) that handles extern of possible
device structs that would come from devicetree.  This removes the need
for DEVICE_DT_DECLARE and DEVICE_DT_INST_DECLARE which we can remove.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>
3 files changed