Makefile: Add a rule 'basic-pbx' that installs the Basic PBX configs

This patch adds a rule for installing the Super Awesome Company based 'Basic
PBX' configuration files. As part of adding this rule, a bit of the content
that makes up installing the configuration files under the 'samples' target
was refactored into a make subroutine for usage by additional later config
make targets.

Change-Id: I6c2e27906f73e2919a2b691da0be20ae70302404