command.h: Add a U_BOOT_LONGHELP macro

In order to be able to discard unused long help texts without further
linker lists, add a macro for defining the long help messages which uses
__maybe_unused.  This allows us to discard them as unreferenced as part
of the link.

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