boot/apex: Restore the APEX boot loader

boot/apex: Restore the APEX boot loader

This is a partial revert of the deletion of the IXP4xx
target: we restore the APEX boot loader so we can use it
for the NSLU2 and related targets.

The APEX upstream is as dead as it gets so I have applied
OpenWrts old patches on top of the never released
v1.6.10 version and forked it into an OpenWrt variant
on GitHub. If the upstream comes back alive I will
happily switch over to it.

The file refers to the external GitHub, I suppose when
integrating this patch the file should be copied to OpenWrts
file repository and the file link changed.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>