Adding NKRO virtual dip-switch, using existing bit in keymap byte.

This takes the last, reserved bit there, but doesn't necessitate
a revision to the magic number because it doesn't alter byte order.
Add reference to NKRO virtual dip-switch to documentation.

common/bootmagic.h

@@ -60,6 +60,9 @@
 #ifndef BOOTMAGIC_KEY_SWAP_BACKSLASH_BACKSPACE
 #define BOOTMAGIC_KEY_SWAP_BACKSLASH_BACKSPACE  KC_BSLASH
 #endif
+#ifndef BOOTMAGIC_HOST_NKRO
+#define BOOTMAGIC_HOST_NKRO              KC_N
+#endif
 
 
 /*