Build System: Create 'make install-configs' target.

This target requires specifying CONFIG_SRC=path_to_configs.  This can be
used to install custom configs for the Asterisk build while still
performing directory replacements on asterisk.conf.

Modify internal INSTALL_CONFIGS so first argument requires full path to
the config sources relative to Asterisk source root.

Change-Id: Idcd841df3c8d5bfe23d566bb9e2e448e9df4f8ab