RPi: just use RPI instead of RPI2.

The reason for this change is that RPi2 and RPi3 are compatible, and
hopefully all differences coming up can be resolved without ifdefs but
at runtime.

cmake/toolchains/Toolchain-arm-linux-gnueabihf-raspbian.cmake

@@ -71,7 +71,7 @@ foreach(tool echo grep rm mkdir nm cp touch make unzip)
 endforeach()
 
 add_definitions(
-	-D __RPI2
+	-D __RPI
 	)
 
 set(LINKER_FLAGS "-Wl,-gc-sections")