Add cut binary to the initramfs

Since the cut -b command is used by parse-zfs.sh,
ensure that it is copied to the initramfs.

Fix spl_hostid when set by cmdline. This follows a
similar logic from the zgenhostid script, using echo
instead of printf.

Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ben Cordero <bencord0@condi.me>
Closes #8197