gummiboot/sd-boot/systemd-boot: rename galore

What used to be gummiboot, was renamed sd-boot when it was merged into
systemd. Let's try to be a bit more consistent with the rest of systemd
and rename it again as follows:

The EFI bootloader is now called 'systemd-bootx64.efi', and its sources are in
'src/boot/efi/'. The drop-in directory where bootctl will find EFI loaders
is now /usr/lib/systemd/boot/efi/.