Fix a bug using ELF as output extension

When overriding the OUTPUT_EXT variable in compilation configuration,
bugs appear:

* The bin variable is not None as the 'elf' string is not interned.
* When that is fixed, the function returns None instead of returning the
path of the ELF file.

Signed-off-by: Hugues de Valon <hugues.devalon@arm.com>

tools/toolchains/__init__.py

@@ -1129,8 +1129,10 @@ class mbedToolchain:
         mkdir(new_path)
 
         filename = name+'.'+ext
+        # Absolute path of the final linked file
+        full_path = join(tmp_path, filename)
         elf = join(tmp_path, name + '.elf')
-        bin = None if ext is 'elf' else join(tmp_path, filename)
+        bin = None if ext == 'elf' else full_path
         map = join(tmp_path, name + '.map')
 
         r.objects = sorted(set(r.objects))
@@ -1154,7 +1156,7 @@ class mbedToolchain:
         self.var("compile_succeded", True)
         self.var("binary", filename)
 
-        return bin, needed_update
+        return full_path, needed_update
 
     # THIS METHOD IS BEING OVERRIDDEN BY THE MBED ONLINE BUILD SYSTEM
     # ANY CHANGE OF PARAMETERS OR RETURN VALUES WILL BREAK COMPATIBILITY