Rename src/boot/efi to just src/boot

Rename src/boot/efi to just src/boot

I very much dislike the approach in which we were mixing Linux and UEFI C code
in the same subdirectory. No code was shared between two environments. This
layout was created in e7dd673d1e0acfe5420599588c559fd85a3a9e8f, with the
justification of "being more consistent with the rest of systemd", but I don't
see how it's supposed to be so.

Originally, when the C code was just a single bootctl.c file, this wasn't so
bad. But over time the userspace code grew quite a bit. With the moves done in
previuos commits, the intermediate subdirectory is now empty except for the
efi/ subdir, and this additional subdirectory level doesn't have a good
justification. The components is called "systemd-boot", not "systemd-efi", and
we can remove one level of indentation.