boot: Improve EFISTUB name and version detection.

Make two minor cosmetic changes to how config_entry_add_linux() extracts
OS information from the embedded /etc/os-release in EFISTUB binaries.
This increases the detail of information available to the user for
EFISTUB entries.

* entry->version, which was previously always null, is now set to the
value of VERSION, or VERSION_ID, or BUILD_ID (preferred in that order).
* entry->title, which was previously set to the value of PRETTY_NAME, can
now fall back to NAME or ID if PRETTY_NAME is not present. In that case,
NAME is preferred over ID.