Allow command code to compile to nothing

When CONFIG_CMDLINE is disabled we need to remove all the command-line
code. Most can be removed by dropping the appropriate linker lists from the
images, but sub-commands must be dealt with specially.

A simple mechanism is used to avoid 'unused static function' errors.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>