efi_loader: Make HII a config option

Heinrich ran into issues with HII and iPXE which lead to #SErrors on
his Odroid-C2 system. We definitely do not want to regress just yet,
so let's not expose the HII protocols by default.

Instead, let's make it a config option that people can play with
This way, we can stabilize the code in tree without breaking any
users.

Once someone figures out, why this breaks iPXE (probably a NULL
dereference), we can enable it by default.

Reported-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>

---

v1 -> v2:

  - Remove HII selftest as well

v2 -> v3:

  - Make config option