Reraising from bug 669230

Since a recent upgrade, plymouth has been falling back to the text splash instead of using the configured graphical boot splash. I tracked this down to failure to include the relevant drm driver into the initramfs image.

The dracut plymouth module, in the installkernel script, greps for drivers
containing the drm initialisation symbol and installs them into the initramfs
image.

However I have in my user environment a fairly complex GREP_OPTIONS containing,
amongst other settings, "--binary-files=without-match". (They tend to be slow
to search and inconvenient to print out to the terminal.) This environment
variable gets propagated to all dracut scripts, causes the grep to fail to find
any drm drivers.

Solution is to have /sbin/dracut "unset GREP_OPTIONS".

(I've also changed from using a globally set environment variable to an alias on grep which sets the environment only for interactive commands which ought to be safer, but it is bad form for system scripts to break under unusual but reasonable user environments.)