Update the grub theme to use the new background and fix up the sizes

Also fix a warning message from GRUB about not finding en_US
Update how grub displays menus

When we only have a single boot-environment, we won't display
the menu, unless the user holds down the SHIFT key. The system
will auto-boot after 2 seconds.

If multiple BE's are found, it will display the menu with a 5 second
timeout, allowing the user to halt and choose another BE.

Suggested by: Jordan Hubbard
Merge branch 'master' of github.com:pcbsd/pcbsd