at91: convert boards to generic build target

Evaluation boards are left in component form to ease flashing
using vendor tooling and instructions. These boards also do
not include the EOF marker in the UBIFS as the bootloaders
are recent and easily upgradeable.

The end product boards use factory.bin images based on the
dts layout and include EOF markers as bootloader UBI support
is not determined.

Signed-off-by: Ben Whitten <ben.whitten@gmail.com>
Signed-off-by: Mathias Kresin <dev@kresin.me>