Linux additions / dkms.conf: don't overwrite MAKE[x] as this has bad side effects

Additions/linux: correctly set the build type (release, debug) when building the kernel modules

Linux Installer/Additions: attempt to properly register the host/guest kernel modules at DKMS and use the setup function of the service script to compile the modules instead of doing this separately in the installer scripts

VBoxGuest (common): building on linux.