boot: Retain ".conf" suffix for loader config IDs.

Change config_entry_add_from_file() so that it does not remove the
`.conf` ending from the filename that it uses for entry->id. This is
necessary because otherwise an EFISTUB binary and a loader config entry
might end up with the same ID, as the loader config IDs previously
didn't include the config filename's .conf extension (see boot.c:1435).
Consider, for instance, an EFISTUB /EFI/Linux/linux.efi and a loader
config /loader/entries/linux.efi.conf; both would have the ID linux.efi.

In addition, update a comment that previously stated that IDs are
non-unique (which is no longer the case, as of this commit).