Local repos with deps

Add support to use the DEPS iso image as install source.
> IMHO this is cleaner and shorter:
> if [ -n "${localreposdir_target}" -a -n "${bindsrc}" ]; then

@jdsn In the case $bindsrc is empty the else branch is executed. That's not what we want.
But what do you think about:
```
if [ -n "${localreposdir_target}" ]; then
    [ -n "${bindsrc}" ] && add_bind_mount "${localreposdir_target}/${bindsrc}" "${targetdir}"
else
    [ -n "${nfssrc}" ] && add_nfs_mount "${nfssrc}" "${targetdir}"
fi
```