[main] Add the "scriptlet" setting

[main] Add the "scriptlet" setting

A scriptlet is a single iPXE command that can be stored in
non-volatile option storage and used to override the default
"autoboot" behaviour without having to reflash the iPXE image.

For example, a scriptlet could contain

    autoboot || reboot

to instruct iPXE to reboot the system if booting fails.

Unlike an embedded image, the presence of a scriptlet does not inhibit
the initial "Press Ctrl-B..." prompt.  This allows the user to recover
from setting a faulty scriptlet.

Originally-implemented-by: Glenn Brown <glenn@myri.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>