Buildarmpkg has a set mirror it uses for creating the rootfs.
This mirror uses the old repo structure and as such will 404 when trying to download the databases.

So make buildarmpkg use the new mirror structure.
And also remove the creation of ARM mirrorlist on the host, since it's not doing anything.
The mirrorlist from the pacman.conf.aaarch64 gets used instead.

Also prepare for some branch support. Default branch is set to stable.
Signed-off-by: Dan Johansen <strit@manjaro.org>