net: Optionally use pxe client arch from variable

The client architecture that we pass to a dhcp server depends on the target
payload that we want to execute. An EFI binary has a different client arch
than a legacy binary or a u-boot binary.

So let's parameterize the pxe client arch field to allow an override via
the distro script, so that our efi boot path can tell the dhcp server that
it's actually an efi firmware.

Signed-off-by: Alexander Graf <agraf@suse.de>