firmware: Implement scanning block

This allows to disable the keyboard. The firmware will never scan
for keys pressed in this mode, and it will powerdown the MCU most of the
time.

firmware/main.c

@@ -1903,6 +1903,13 @@ void main(void)
 		}
 #endif
 
+		// do nothing if scanning is blocked
+		if (cfg & REG_SYS_CONFIG_SCAN_BLOCK) {
+			if (scan_active)
+				keyscan_idle();
+			continue;
+		}
+
 		// if active scanning is not active and port 6 change was
 		// detected, and some key is still pressed, enter active
 		// scanning mode