De-automate install and remove of gummiboot

If we look at how the GRUB ebuild works, it does
not explicitly install the boot loader onto the disk.
Instead, the user must do this manually.
I assume this is for a number of reasons, one of which
being that the user should have explicit control over
such a key part of the system. The user may also have
various boot loaders emerged, but not all of them installed
at once. We therefore do not want to interfere with this.

Instructions have been added to the ebuild in the form of
elog messages regarding how to maintain a gummiboot installation.