"boot" command to attempt booting from within the command shell, fall back
to shell if boot fails for any reason.

the option config UI.

Introduce new {show,set,clear}_setting() that take a struct setting *
rather than a const char *.

set_setting() handles calling clear_setting() when appropriate, so that
individual setting types don't have to check for empty strings.

memory" and "no space left for DHCP options"

command is linked in.

non-ANSI-capable consoles.

in our commands.

completely forgot I'd written ages ago.

device.

Separated the mechanisms of non-volatile storage access and non-volatile
stored options.

for realloc(0) being a valid way to free memory.

with octal yet, but we can probably live without that.

defined in vsprintf.h.  (This may change, since vsprintf.h is a
non-standard name, but for now it's the one to use.)

There should be no need to include vsprintf.h just for DBG() statements,
since include/compiler.h forces it in for a debug build anyway.