Set flags for building the proper DriverFramework target

The DF_TARGET cmake variable needs to be set to (linux, darwin, qurt, nuttx)
The define __DF_LINUX or __DF_QURT needs to be set.
The __DF_DARWIN target is untested

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

cmake/nuttx/px4_impl_nuttx.cmake

@@ -429,6 +429,7 @@ function(px4_os_add_flags)
 		)
 	set(added_definitions
 		-D__PX4_NUTTX
+		-D__DF_NUTTX
 		)
 	set(added_c_flags
 		-nodefaultlibs