Fix build

connectivity/drivers/802.15.4_RF/CMakeLists.txt

@@ -1,23 +1,43 @@
 # Copyright (c) 2020 ARM Limited. All rights reserved.
 # SPDX-License-Identifier: Apache-2.0
 
-add_library(mbed-802.15.4-rf STATIC EXCLUDE_FROM_ALL)
+# The 'mbed-802.15.4-rf' target is lazily created because there might not be any subdirectories needing it at all.
+macro(create_802_15_4_target)
+	if(NOT TARGET create_802_15_4_target)
+		add_library(mbed-802.15.4-rf STATIC EXCLUDE_FROM_ALL)
+
+		# Nanostack drivers always require Mbed RTOS
+		target_link_libraries(mbed-802.15.4-rf PUBLIC mbed-core-flags mbed-rtos-flags)
+
+		target_link_libraries(mbed-nanostack
+		    INTERFACE
+		        mbed-802.15.4-rf
+		)
+	endif()
+endmacro()
 
-# Nanostack drivers always require Mbed RTOS
-target_link_libraries(mbed-802.15.4-rf PUBLIC mbed-core-flags mbed-rtos-flags)
 
 if("Freescale" IN_LIST MBED_TARGET_LABELS)
+	create_802_15_4_target()
     add_subdirectory(TARGET_Freescale)
-    add_subdirectory(mcr20a-rf-driver)
 elseif("Silicon_Labs" IN_LIST MBED_TARGET_LABELS)
+	create_802_15_4_target()
     add_subdirectory(TARGET_Silicon_Labs)
-elseif("STM" IN_LIST MBED_TARGET_LABELS)
-    add_subdirectory(stm-s2lp-rf-driver)
 endif()
 
-add_subdirectory(atmel-rf-driver)
+if("COMPONENT_MICROCHIP_AT86RF=1" IN_LIST MBED_TARGET_DEFINITIONS)
+	create_802_15_4_target()
+	add_subdirectory(COMPONENT_MICROCHIP_AT86RF)
+endif()
+
+if("COMPONENT_NXP_MCR20A=1" IN_LIST MBED_TARGET_DEFINITIONS)
+	create_802_15_4_target()
+	add_subdirectory(COMPONENT_NXP_MCR20A)
+endif()
+
+if("COMPONENT_STM_S2_LP=1" IN_LIST MBED_TARGET_DEFINITIONS)
+	create_802_15_4_target()
+	add_subdirectory(COMPONENT_STM_S2_LP)
+endif()
+
 
-target_link_libraries(mbed-nanostack
-    INTERFACE
-        mbed-802.15.4-rf
-)