Put /boot/efi back in mtree

[nwhitehorn]

Summary

The attached patch puts /boot/efi back in mtree, so it is created on systems without an ESP as well as systems with an ESP (I still don't really understand why we want this...). To make installations work so that only real ESPs get set up with the loader and efibootmgr, the logic in the installer is switched from whether /boot/efi exists (which it now always does) to whether it is present as a mountpoint in the in-progress fstab.

NOTE: I have not tested this fully yet, but it takes long enough to do end-to-end build and test for installer changes that I'm putting this up for review now to save time. It's a 2-line diff anyway, so I'm guessing most of the discussion will be on concept rather than any bugs that might exist.

[rgrimes]

This is a move in the right directions, at least this is state created by the installer.

[bdragon]

Confirmed that I can still install on powerpc64le in qemu with this change, and that the empty /boot/efi directory exists again.

[nwhitehorn]

In D29068#651177, @rgrimes wrote:

This is a move in the right directions, at least this is state created by the installer.

It was always state created by the installer, for the record. It's just different state now (whether it make a directory earlier vs. whether it added that directory to /etc/fstab after making it).