Add Pican2 support

In order to make Pican 2 work, we :
- add mcp2515.dto to the overlay list
- add a variable to enable it in local.conf
- create a udev rule to mount interface

http://skpang.co.uk/catalog/pican2-canbus-board-for-raspberry-pi-23-p-1475.html

Signed-off-by: Fabien Lahoudere <fabien.lahoudere@collabora.com>

docs/extra-build-config.md

@@ -243,6 +243,15 @@ When using device tree kernels, set this variable to enable the 802.15.4 hat:
 
 See: <https://openlabs.co/OSHW/Raspberry-Pi-802.15.4-radio>
 
+## Enable CAN with Pican2
+
+In order to use Pican2 CAN module, set the following variables:
+
+	ENABLE_SPI_BUS = "1"
+	ENABLE_CAN = "1"
+
+See: <http://skpang.co.uk/catalog/pican2-canbus-board-for-raspberry-pi-23-p-1475.html>
+
 ## Manual additions to config.txt
 
 The `RPI_EXTRA_CONFIG` variable can be used to manually add additional lines to