fix cold bootup keyboard issue on the f256k

KBD_OPTICAL (0xf01dc0) is only "valid" on F256K2 hardware. i would
assume that this value is unknown on non-F256K2 hardware, thus sometimes
you might read some garbage value from here perhaps?

what i observed on my f256k was that always on cold bootup (that is,
booting from flash) the keyboard would never work. hitting reset or
booting from a ram build would not exhibit the same problem.

sometimes (somewhat more rarely) kbd_init() would lock up at some
point, presumably when it was trying to initialize/scan the non-existent
optical keyboard.

so far this also *seems* to resolve intermittent issues with reads to
the SD card on my f256k locking up. best not to read from those new
F256K2 registers on non-K2 devices!

src/dev/kbd_f256k.c

@@ -14,6 +14,7 @@
 #include <stdint.h>
 #include <stdio.h>
 
+#include "features.h"
 #include "interrupt.h"
 #include "log.h"
 #include "ring_buffer.h"
@@ -330,11 +331,20 @@ bool kbd_break() {
  *
  */
 short kbd_sc_init() {
+	// NOTE: Reading from the new optical keyboard IO registers on non-F256K2 
+	//       devices causes odd and sometimes intermittent issues not limited to
+	//       the keyboard not working at all right after a cold bootup from flash.
+	//       Possibly this can be addressed with future FPGA updates for 
+	//       non-F256K2 devices, but for now we want to avoid this situation.
+#if HAS_OPTICAL_KBD_SUPPORT
 	if (KBD_OPTICAL->status & KBD_OPT_STAT_MECH) {
 		is_optical = false;
 	} else {
 		is_optical = true;
 	}
+#else
+	is_optical = false;
+#endif
 
 	// Initialize VIA0 -- we'll just read from PB7
 	via0->ddra = 0x00;

src/include/features.h

@@ -46,6 +46,10 @@
 	#define HAS_IEC 1
 #endif
 
+#if MODEL == MODEL_FOENIX_F256K2 || MODEL == MODEL_FOENIX_F256K2E
+	#define HAS_OPTICAL_KBD_SUPPORT 1
+#endif
+
 
 /* Defaults -----------------------------------------------------------------*/
 
@@ -93,4 +97,8 @@
 	#define HAS_IEC 0
 #endif
 
+#ifndef HAS_OPTICAL_KBD_SUPPORT
+	#define HAS_OPTICAL_KBD_SUPPORT 0
+#endif
+
 #endif