feat(boards): Add BDN9 Rev2 board.

* Onboard stm32f072.
* 3 possible encoder positions.
* Underglow/per-key not yet support.

app/boards/arm/bdn9/Kconfig.defconfig

@@ -0,0 +1,17 @@
+# keeb.io BDN9 board configuration
+
+# Copyright (c) 2020 Pete Johanson
+# SPDX-License-Identifier: MIT
+
+if BOARD_BDN9
+
+config BOARD
+	default "bdn9_rev2"
+
+config ZMK_KEYBOARD_NAME
+	default "BDN9 Rev2"
+
+config ZMK_USB
+	default y
+
+endif # BOARD_BDN9