Allow setting dist and mock configuration in Makefile

Allow setting dist and mock configuration in Makefile

Curently, the dist and mock configurations are hardcoded in the
Makefile to be el8 and epel-7-x86_64, respectively. This commit
allows the user to override those settings using the DIST and
MOCK_CONFIG environment variables, falling back to the current
defaults if not set.

This provides additional flexibility when building the RPM directly
from the repository.

Signed-off-by: Javier Peña <jpena@redhat.com>