busybox: Provide /bin/ash when usrmerge is enabled

busybox: Provide /bin/ash when usrmerge is enabled

When usrmerge is enabled, scripts that were explicitly using #!/bin/ash
will cause a QA Error like the following:

QA Issue: bar.sh contained in package foo requires /bin/ash,
but no providers found in RDEPENDS_foo? [file-rdeps].

It seems perfectly acceptable for scripts to use /bin/ash so provide
it along with /bin/sh.

Signed-off-by: Wes Lindauer <wesley.lindauer@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>