Add fonts to the EFI images

The previous security updates disallowed loading unsigned fonts when
in SB mode. To make things work again:

 * Embed the "unicode" font into the embedded memdisk image so it can
   be loaded.
 * Add the memdisk to our normal grubx64.efi loader too
 * Add a patch from Chris Coulson to make the font loader look for
   fonts in the memdisk whenever they're loaded.

Closes: #1024395, #1025352, #1024447