Abstract check_executable added

tools/toolchains/__init__.py

@@ -1130,6 +1130,13 @@ class mbedToolchain:
         # User could have specified a path that exists but does not contain exe
         return exists(subdir) or exists(subdir +'.exe')
 
+    @abstractmethod
+    def check_executable(self):
+        """Returns True if the executable (armcc) location specified by the
+         user exists OR the executable can be found on the PATH.
+         Returns False otherwise."""
+        raise NotImplemented
+
     @abstractmethod
     def get_config_option(self, config_header):
         """Generate the compiler option that forces the inclusion of the configuration