Some shuffling around of the image management code; this needs tidying up.

"kernel".

can't modify it).

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

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.