Allow a user-overridable setting 'PKG_CMD' to control the command used

Allow a user-overridable setting 'PKG_CMD' to control the command used
to create a repo during 'make packages'

This would have been useful for a situation I found myself in where
pkg(8) had been upgraded to a version that wanted the FBSD_1.5 ABI
version but libc.so.7 had not been upgraded, and only provided
FBSD_1.4. I found I needed to update libc in order to run pkg, and I
also needed to use pkg to update libc... Which is why pkg-static
exists, but there's currently no way to tell the build system to use
pkg-static instead of pkg.

This creates a variable PKG_CMD, default value 'pkg', that can be
overridden from the command line.

Reviewed by:	gjb
Approved by:	gjb
Differential Revision:	https://reviews.freebsd.org/D8120