Fix ebuild to install, update and uninstall cleanly.

Aside from the above, the ebuild also assumes that the ESP
is mounted at /boot rather than /boot/efi. This is in accordance
with upstream's recommendations. The user will be warned if this
may not be the case.

The ebuild now also handles the installing, updating and uninstalling
of gummiboot from within the ebuild, rather than requiring
exterior user action.