Put both files in the same directory and make makepkg-arm executable, then use makepkg-arm instead of makepkg, cli parameters are passed through if given

makepkg-arm

#!/bin/bash
# makepkg-arm script
DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
makepkg --config "$DIR/makepkg-arm.conf" "$@"

makepkg-arm.conf

#
# makepkg-arm.conf
# make sure this is in the same dir as makepkg-arm or adjust makepkg-arm
#
source /etc/makepkg.conf

export PATH=$PATH:/opt/x-tools/arm-unknown-linux-gnueabi/bin
export CROSS_COMPILE=arm-unknown-linux-gnueabi-

CARCH="arm"
CHOST="arm-unknown-linux-gnueabi"
export ARCH='arm'

#-- Make Flags: change this for DistCC/SMP systems
MAKEFLAGS="-j4"

# vim: set ft=sh ts=2 sw=2 et: