hooks/growcrypt: copy_exec grep, flock

I haven't tested these changes.

Resolves this concern listed on first boot of the installer image:

Begin: Running /scripts/init-premount ... /scripts/init-premount/growcrypt: 22: /scripts/init-premount/growcrypt: grep: not found

Resolves this concern on first boot of the installed (installer-produced) image:

failed [flock:127] flock -x 9
/sbin/growpart: line 714: flock: not found
FAILED: Error while obtaining exclusive lock on /dev/mmcblk0