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

cmake/posix/px4_impl_posix.cmake

@@ -171,6 +171,7 @@ if(UNIX AND APPLE)
         set(added_definitions
 		-D__PX4_POSIX
 		-D__PX4_DARWIN
+		-D__DF_DARWIN
 		-DCLOCK_MONOTONIC=1
 		-Dnoreturn_function=__attribute__\(\(noreturn\)\)
 		-include ${PX4_INCLUDE_DIR}visibility.h
@@ -185,6 +186,7 @@ else()
         set(added_definitions
 		-D__PX4_POSIX
 		-D__PX4_LINUX 
+		-D__DF_LINUX 
 		-DCLOCK_MONOTONIC=1
 		-Dnoreturn_function=__attribute__\(\(noreturn\)\)
 		-include ${PX4_INCLUDE_DIR}visibility.h