When populating the dvd/packages/ directory, create
a symlink to All/pkg-*.txz in the Latest/ directory.

This allows 'pkg bootstrap' to work out-of-box if
the REPOS_DIR environment is properly set.

Tested on:	stable/10@r271848
MFC after:	3 days
X-MFC-10.1:	yes
Sponsored by:	The FreeBSD Foundation