One of the already implemented options in release/Makefile is NOSRC. When
it's defined, installation image is shipped without source distribution
(src.txz)

Add the hability of defining NOSRC in release.conf and pass it to
'make release' argument

Approved by: gjb
Sponsored by: Rubicon Communications (Netgate)
Differential Revision: https://reviews.freebsd.org/D6710