build: Define DECL_CONSTANT mechanism for defining exported constants

Add a DECL_CONSTANT macro to allow the firmware to define constants
that are to be exported to the host during the "identify" phase.  This
replaces the existing hardcoded mechanism of scanning the Kconfig
header file for certain constants.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/console.py

@@ -27,7 +27,7 @@ class KeyboardReader:
         self.reactor.unregister_timer(self.connect_timer)
         return self.reactor.NEVER
     def update_evals(self, eventtime):
-        f = self.ser.msgparser.config.get('CLOCK_FREQ', 1)
+        f = int(self.ser.msgparser.config.get('CLOCK_FREQ', 1))
         c = self.ser.get_clock(eventtime)
         self.eval_globals['freq'] = f
         self.eval_globals['clock'] = int(c)