configs/pcengines_apu2: new defconfig

configs/pcengines_apu2: new defconfig

The apu2 by PC Engines is a 64 bit single board computer with coreboot
BIOS. It only has serial output, but no graphics chip. With its 2–4
ethernet ports, it is often used to build a firewall or router.

The provided defconfig configures busybox and isolinux. It will output
a hybrid ISO image that can be written to a USB stick or burned to a
CD.

Configuration based on a blogpost by Tony Arkles:

http://www.better-bsp.com/blog/2017/03/02/buildrooting-for-apu2/

Signed-off-by: Danilo Bargen <mail@dbrgn.ch>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>