[settings] Introduce the generalised concept of a numeric setting

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[cmdline] Add "inc" command

The "inc" command allows the numeric value of a setting to be
incremented, allowing for the construction of simple loops within an
iPXE script.

Signed-off-by: Michael Brown <mcb30@ipxe.org>