build.install: support RHEL

Building RHEL image is now supported. To do that you need to have
access to a rhel-source repository.

To buil such an image you have to fill the environment variable
RHEL_REPO with your repo before calling ``make base``. A sample
invocation could be:

make base DIST=redhat DVER=R64 RHEL_REPO=http://1.2.3.4/rhel64/