The for loop didn't find $curdir, because it was set _after_ the directory has
been changed. The variable is now called $srcroot and is set before the
installPhase is changing directories.

The scripts/depmod.sh checks whether the path in $DEPMOD is executable and only
executes it if that's the case. So, by setting DEPMOD to "/do_not_use_depmod"
the destination path doesn't exist _and_ thus isn't executable aswell.

First of all, modules won't install when there is no "make modules" prior to it,
so we're doing this now with a new function called forEachModule, so we can
avoid duplication as much as possible.

In addition this sets $sourcedir to the current directory of the configurePhase,
so we're able to find the source tree later on, after several chdir()s.

The "include/generated" stuff is already added to the search path by the kernel
modules build system, so no need to hack it in like this.

We're the host so we don't need the kernel modules for the guest additions, as
they're build with in a separate derivation anyway.

Copy all symbol versions from the ones generated in the vboxdrv build.

Thanks to @shlevy for mentioning this.