busybox: fix a sh link wrong

When both bash and busybox be installed, without ash support
in busybox,if bash is installed before busybox in the final stage,
even if ALTERNATIVE_PRIORITY of bash > ALTERNATIVE_PRIORITY of busybox,
the symlink from /bin/sh to bash can be yet overwritten by busybox.

Signed-off-by: Wenlin Kang <wenlin.kang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>