Enh user build

Hiya

These patches allow the freebsd-wifi-build to be run completely as a normal user, without having to become root.
It also allows additional items to be added to src.conf without patching freebsd-wifi-build.
I also added instructions on how to use the build to include ports into the mfsroot.

If you like I can think of several further improvements I could do, including scripts to resolve dependencies and build ports.

Tested against freebsd-release-10.1.0 and -CURRENT
On 10.0.0 I did get it working but I had to hack a few things not in this diff and I haven't needed to go back and use 10.0 again/