main/[yd]ash: fix install of -binsh
When installing [yd]ash-binsh but [yd]ash is not installed the previous binsh
may be removed early leaving [yd]ash.post-install without a shell to run.
Using the shell itself also resolves a recursive dependency between [yd]ash
and [yd]ash-binsh.
Additional dependency on busybox is needed when creating a rootfs to
ensure the add-shell command is actually available when running the
[yd]ash.post-install script.
(Extracted from !65301 (comment 403066))