meta-asrock: Add bios-update infrastructure

On all supported ASRock platforms the BIOS (host firmware) update
mechanism involves setting some GPIOs, binding a driver for an MTD flash
device, writing the image to the flash, and then undoing the setup steps
in reverse order.  The exact GPIOs involved (and some parameters of the
firmware image) vary a bit between systems; the platform-specific
differences are distilled out into some config parameters loaded from
/etc/default/bios-update.

Signed-off-by: Zev Weiss <zev@bewilderbeest.net>
Co-developed-by: Olivier Faurax <olivier.faurax@eu.equinix.com>
Change-Id: I4ecf93a73359063e3639895b078057a637dff452