Busybox no longer has CONFIG_BASH since we are deploying bash on most
boards. We also should clearly indicate which scripts cannot use
bashisms.
Change shebang in x230-flash.init, t430-flash.init, flash.sh to
/bin/ash. Execute /bin/sh for interactive shells.
Move key functions needed by those scripts to initrd/etc/ash_functions.
Source ash_functions instead of functions in those scripts, so any
bashisms in other functions won't break parsing of the script in ash.
Signed-off-by: Jonathon Hall <jonathon.hall@puri.sm>