Do not run VirtualBox setup under enabled Secure Boot to avoid errors and startup...

The VirtualBox package from upstream isn't signed for usage with Secure
Boot with the Debian kernel.

When booting with Secure Boot enabled, then upstream's vboxdrv.service with
its vboxdrv.sh executes all kind of Secure Boot related magic like:

| /usr/bin/perl -w /usr/share/debconf/frontend /usr/sbin/update-secureboot-policy --new-key

This fails and causes a noticeable delay during bootup.  Therefore skip
execution of VirtualBox setup within our config_virtualbox_setup() when
detecting enabled Secure Boot mode, at least until we've a better solution
for this.

While doing so, move detection of enabled Secure Boot mode into a helper
function to avoid DRY code.

Thanks: Ralf Moll for the bugreport