Upload: Fix mbed_adjust_upload_debug_commands for multiple targets (#391)

The purpose of mbed_adjust_upload_debug_commands() is to adjust
first version of MBED_UPLOAD_LAUNCH_COMMANDS. For multiple executalbe
cmake targets, MBED_UPLOAD_LAUNCH_COMMANDS becomes not fresh and may
cause error. To overcome this, one new cmake cache variable
MBED_UPLOAD_LAUNCH_COMMANDS_BAK is introduced to save first version of
MBED_UPLOAD_LAUNCH_COMMANDS.

Author: ccli8

tools/cmake/UploadMethodManager.cmake

@@ -12,8 +12,14 @@
 # NOTE: Place at the very start so that it can override by the below loaded
 #       upload method if need be.
 function(mbed_adjust_upload_debug_commands target)
-    # MBED_UPLOAD_LAUNCH_COMMANDS defined?
-    if(NOT DEFINED MBED_UPLOAD_LAUNCH_COMMANDS)
+    # MBED_UPLOAD_LAUNCH_COMMANDS_BAK = first version of MBED_UPLOAD_LAUNCH_COMMANDS
+    if(DEFINED MBED_UPLOAD_LAUNCH_COMMANDS_BAK)
+        # Need first version for fresh adjust
+        set(MBED_UPLOAD_LAUNCH_COMMANDS ${MBED_UPLOAD_LAUNCH_COMMANDS_BAK})
+    elseif(DEFINED MBED_UPLOAD_LAUNCH_COMMANDS)
+        # No FORCE for saving first version only
+        set(MBED_UPLOAD_LAUNCH_COMMANDS_BAK ${MBED_UPLOAD_LAUNCH_COMMANDS} CACHE INTERNAL "")
+    else()
         return()
     endif()