CMake: Image format based on MBED_OUTPUT_EXT

Requires: ARMmbed/mbed-tools#242

The optional `OUTPUT_EXT` field in Mbed OS `targets.json` specifies
the format of the output image. This is important because
* some targets only support one image format
* each post-binary hook outputs one image only

This avoids confusion of having both .hex and .bin images generated
while only one of them is usable.

Author: LDong-Arm

CMakeLists.txt

@@ -144,21 +144,33 @@ target_link_libraries(mbed-core INTERFACE ${MBED_TARGET_CONVERTED})
 #
 function(mbed_generate_bin_hex target)
     get_property(elf_to_bin GLOBAL PROPERTY ELF2BIN)
-    if(MBED_TOOLCHAIN STREQUAL "GCC_ARM")
-        set(CMAKE_POST_BUILD_COMMAND
-            COMMAND ${elf_to_bin} -O binary $<TARGET_FILE:${target}> ${CMAKE_CURRENT_BINARY_DIR}/${target}.bin
-            COMMAND ${CMAKE_COMMAND} -E echo "-- built: ${CMAKE_CURRENT_BINARY_DIR}/${target}.bin"
-            COMMAND ${elf_to_bin} -O ihex $<TARGET_FILE:${target}> ${CMAKE_CURRENT_BINARY_DIR}/${target}.hex
-            COMMAND ${CMAKE_COMMAND} -E echo "-- built: ${CMAKE_CURRENT_BINARY_DIR}/${target}.hex"
-        )
+    if (MBED_TOOLCHAIN STREQUAL "GCC_ARM")
+        if (MBED_OUTPUT_EXT STREQUAL "" OR MBED_OUTPUT_EXT STREQUAL "bin")
+            list(APPEND CMAKE_POST_BUILD_COMMAND
+                COMMAND ${elf_to_bin} -O binary $<TARGET_FILE:${target}> ${CMAKE_CURRENT_BINARY_DIR}/${target}.bin
+                COMMAND ${CMAKE_COMMAND} -E echo "-- built: ${CMAKE_CURRENT_BINARY_DIR}/${target}.bin"
+            )
+        endif()
+        if (MBED_OUTPUT_EXT STREQUAL "" OR MBED_OUTPUT_EXT STREQUAL "hex")
+            list(APPEND CMAKE_POST_BUILD_COMMAND
+                COMMAND ${elf_to_bin} -O ihex $<TARGET_FILE:${target}> ${CMAKE_CURRENT_BINARY_DIR}/${target}.hex
+                COMMAND ${CMAKE_COMMAND} -E echo "-- built: ${CMAKE_CURRENT_BINARY_DIR}/${target}.hex"
+            )
+        endif()
     elseif(MBED_TOOLCHAIN STREQUAL "ARM")
         get_property(mbed_studio_arm_compiler GLOBAL PROPERTY MBED_STUDIO_ARM_COMPILER)
-        set(CMAKE_POST_BUILD_COMMAND
-            COMMAND ${elf_to_bin} ${mbed_studio_arm_compiler} --bin  -o ${CMAKE_CURRENT_BINARY_DIR}/${target}.bin $<TARGET_FILE:${target}>
-            COMMAND ${CMAKE_COMMAND} -E echo "-- built: ${CMAKE_CURRENT_BINARY_DIR}/${target}.bin"
+        if (MBED_OUTPUT_EXT STREQUAL "" OR MBED_OUTPUT_EXT STREQUAL "bin")
+            list(APPEND CMAKE_POST_BUILD_COMMAND
+                COMMAND ${elf_to_bin} ${mbed_studio_arm_compiler} --bin  -o ${CMAKE_CURRENT_BINARY_DIR}/${target}.bin $<TARGET_FILE:${target}>
+                COMMAND ${CMAKE_COMMAND} -E echo "-- built: ${CMAKE_CURRENT_BINARY_DIR}/${target}.bin"
+            )
+        endif()
+        if (MBED_OUTPUT_EXT STREQUAL "" OR MBED_OUTPUT_EXT STREQUAL "hex")
+            list(APPEND CMAKE_POST_BUILD_COMMAND
             COMMAND ${elf_to_bin} ${mbed_studio_arm_compiler} --i32combined  -o ${CMAKE_CURRENT_BINARY_DIR}/${target}.hex $<TARGET_FILE:${target}>
             COMMAND ${CMAKE_COMMAND} -E echo "-- built: ${CMAKE_CURRENT_BINARY_DIR}/${target}.hex"
-        )
+            )
+        endif()
     endif()
     add_custom_command(
         TARGET