Extend macros with those found in MACROS.txt

Otherwise the supplied macros from the function arguments will be dropped.

Author: theotherjimmy

mbed/mbed.py

@@ -1765,7 +1765,7 @@ def get_macros(self, more_macros=None):
         # backwards compatibility with old MACROS.txt file
         if os.path.isfile('MACROS.txt'):
             with open('MACROS.txt') as f:
-                macros = f.read().splitlines()
+                macros.extend(f.read().splitlines())
         return macros