feat(boards): Add BDN9 Rev2 board.

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

app/boards/arm/bdn9/README.md

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+# Building ZMK for the BDN9
+
+Some general notes/commands for building standard BDN9 layouts from the assembly documentation.
+
+## Standard Build
+
+```
+west build -p -d build/bdn9 --board bdn9_rev2
+```
+
+## Encoder Notes
+
+If you built your BDN9 with encoders, you'll need to change the following in your local BDN9 config or add them to the end of the file.
+
+```
+CONFIG_EC11=y
+CONFIG_EC11_TRIGGER_GLOBAL_THREAD=y
+```
+
+Then, you'll want to uncomment the necessary encoder lines in your `bdn9_rev2.keymap`:
+
+```
+&sensors {
+     status = "okay";
+     sensors = <&left_encoder &mid_encoder &right_encoder>;
+};
+
+&left_encoder { status = "okay"; };
+&mid_encoder { status = "okay"; };
+&right_encoder { status = "okay"; };
+```
+
+And then add the correct `sensor-bindings` array to each keymap layer, e.g.:
+
+```
+sensor-bindings = <&inc_dec_kp PG_UP PG_DN &inc_dec_kp M_VOLU M_VOLD &inc_dec_kp C_PREV C_NEXT>;
+```