use ash for non posix shell code
Currently mkinitfs breaks when not using busybox-binsh and therefor the aport has a hard dependency on it0. This allows removing the 'quick workaround'1.
The shell opened in single user mode is not changed though. So for example using dash as a single user mode shell still requires adding /usr/bin/dash to the mkinitfs manually.
The emergency shell is always the busybox shell.