tools: mbed sdk boot should be separate object file

This was probably deleted during the rebase, as mbed sdk boot needs to be
own object file due to using weak functions.

Author: 0xc0170

tools/build_api.py

@@ -1061,7 +1061,7 @@ def build_mbed_libs(target, toolchain_name, verbose=False,
         #                       weak SDK functions
         #   - mbed_main.o: this contains main redirection
         separate_names, separate_objects = ['mbed_retarget.o', 'mbed_board.o',
-                                            'mbed_overrides.o', 'mbed_main.o'], []
+                                            'mbed_overrides.o', 'mbed_main.o', 'mbed_sdk_boot.o'], []
 
         for obj in objects:
             for name in separate_names: