CMake: Fix mbed-802.15.4-rf driver

mbed-nanostack should depend on mbed-802.15.4-rf to avoid a linker error
with an undefined reference to `NanostackRfPhy::get_default_instance()`.
The error occurs when `device_has: 802_15_4_PHY` is defined and the
consumer depends on mbed-nanostack in their CMakeLists.txt. Previously
we linked mbed-nanostack to mbed-802.15.4-rf, so mbed-802.15.4-rf's
usage requirements weren't forwarded to consumers who depended on
mbed-nanostack.

With the previous configuration, the consumer would have to depend on
mbed-802.15.4-rf directly to avoid an issue. This seems like a layering
violation: it appears that mbed-nanostack is "the API" and
mbed-802.15.4-rf is one of several possible implementations which are
selected based on configuration macros.

This commit changes the flow of dependencies so that mbed-nanostack ends
up with the correct symbol definitions.

Author: rwalton-arm

connectivity/drivers/802.15.4_RF/CMakeLists.txt

@@ -12,7 +12,7 @@ endif()
 
 add_subdirectory(atmel-rf-driver)
 
-target_link_libraries(mbed-802.15.4-rf
+target_link_libraries(mbed-nanostack
     INTERFACE
-        mbed-nanostack
+        mbed-802.15.4-rf
 )