Add ability to set build and image mirrors separately
Created by: vignanl
Currently if a local Debian mirror such as apt-cacher-ng is set during build, the resulting images will also contain the same mirror which is not desirable.
This patch separates the mirror used to build the image from the mirror used inside the final image. By default build mirror and image mirror are the same. However, build mirror can be specified by passing an environment variable BUILD_MIRROR to the make command.
make -e BUILD_MIRROR=<build_mirror>
user concern to set mirrors.