upstart: Fix update-alternatives setup

:Release Notes:
sysvcompat package was removed from upstart 1.8 version, but
alternatives now included directly in ${PN} weren't added to
ALTERNATIVE_${PN}, so that u-a wasn't used for e.g. telinit
and sysvinit and upstart were conflicting.

:Detailed Notes:
Fixes do_rootfs issue like:
| Collected errors:
|  * check_data_file_clashes: Package sysvinit wants to install file
        rootfs/sbin/telinit
| 	But that file is already provided by package  * upstart
|  * opkg_install_cmd: Cannot install package sysvinit-dbg.

Drop unused PRIORITY variable, because people can assume that it has
something to do with update-alternatives.

:Testing Performed:
Only build tested and confirmed that telinit is using u-a.

:QA Notes:
No user visible change in image.

:Issues Addressed:
[GF-2635] Upgrade to Upstart 1.8

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <martin.jansa@lge.com>
Change-Id: Ib1c9b582980fc65734cffec059de3322f9445be6
Reviewed-on: https://g2g.palm.com/3307
Reviewed-by: Martin Jansa <Martin.Jansa@lge.com>
Tested-by: Martin Jansa <Martin.Jansa@lge.com>
Reviewed-by: Ed Chejlava <ed.chejlava@lge.com>
Reviewed-by: Susan Montooth <susan.montooth@lge.com>
Tested-by: Susan Montooth <susan.montooth@lge.com>