Add NIBBLE shield

+ Added required files for nullbits NIBBLE

app/boards/shields/nibble/Kconfig.defconfig

@@ -0,0 +1,13 @@
+# Copyright (c) 2020 The ZMK Contributors
+# SPDX-License-Identifier: MIT
+
+if SHIELD_NIBBLE
+
+config ZMK_KEYBOARD_NAME
+	default "NIBBLE_BLE"
+
+config ZMK_USB
+	default y
+
+endif
+