Add support for R9M module (#5101)

* Implement R9M protocol according to PXX spec 1.7

Some of the things here are guesswork as I don’t have a R9M at hand. Also Companion support for the extra R9M options is not implemented yet.

* Fix build errors

radio/src/gui/gui_common_arm.cpp

@@ -705,3 +705,18 @@ void editStickHardwareSettings(coord_t x, coord_t y, int idx, event_t event, Lcd
   else
     lcdDrawMMM(x, y, flags);
 }
+
+const char* getR9MPowerString(int power)
+{
+  switch (power) {
+    case 0:
+      return " 10 mW";
+    case 1:
+      return "100 mW";
+    case 2:
+      return "500 mW";
+    case 3:
+    default:
+      return "  1 W";
+  }
+}