bash: Use u-a for /bin/bash binary conflicting with busybox

:Release Notes:
Use bash applet provided by busybox even when proper bash package is
installed.

:Detailed Notes:
busybox applet was enabled in this commit:
  commit 15cdba524b55c5d8f6d616597faf48fa67637395
  Author: Iyad Qumei <iyad.qumei@lge.com>
  Date:   Mon Nov 18 14:53:07 2013 -0800
  Subject: webos-image: Switch to non-GPLv3 versions of bash, gzip,
           readline & tar

Since then there is following error in devel images's log.do_rootfs:
update-alternatives: Error: not linking rootfs//bin/bash to
  /bin/busybox since rootfs//bin/bash exists and is not a link
and different implementation is used in end-user (busybox) and devel
images (bash proper). Also simplify existing change to keep /bin/sh
pointing to busybox's ash aplet, no need to reset all variables, just
lower the priority.

:Testing Performed:
Only build tested and verified that log.do_rootfs doesn't show errors
about u-a and that the u-a links point to the same tools as before.

:QA Notes:
No functional change to end-user image, devel images now have /bin/bash
from busybox.

:Issues Addressed:
[GF-54122] Busybox applets conflicting with proper tools

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <martin.jansa@lge.com>
Change-Id: Ib1f1d1d37916c4958c99b2a4e5c602e28ebb7468
Reviewed-on: https://g2g.palm.com/4165
Reviewed-by: DCO Verification
Build: Martin Jansa <Martin.Jansa@lge.com>
Reviewed-by: Martin Jansa <Martin.Jansa@lge.com>
Reviewed-by: Build Verification
Tested-by: Martin Jansa <Martin.Jansa@lge.com>
Reviewed-by: Susan Montooth <susan.montooth@lge.com>