Update mbed-cli to allow different configured path for mbed-os

In not every project structure it makes sense to have the mbed-os directory at the root of your project structure.
This change allows you to configure a custom path for the mbed-os directory in your .mbed configuration

Author: daid

mbed/mbed.py

@@ -1384,6 +1384,8 @@ def unset_root(self, path=None):
 
     # Gets mbed OS dir (unified)
     def get_os_dir(self):
+        if self.get_cfg('MBED_OS_DIR', None) is not None:
+            return self.get_cfg('MBED_OS_DIR')
         if os.path.isdir(os.path.join(self.path, 'mbed-os')):
             return os.path.join(self.path, 'mbed-os')
         elif self.name == 'mbed-os':