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>

include/command.h

@@ -328,6 +328,9 @@ int cmd_source_script(ulong addr, const char *fit_uname, const char *confname);
 # define _CMD_HELP(x)
 #endif
 
+#define U_BOOT_LONGHELP(_cmdname, text)					\
+	static __maybe_unused const char _cmdname##_help_text[] = text
+
 #define U_BOOT_SUBCMDS_DO_CMD(_cmdname)					\
 	static int do_##_cmdname(struct cmd_tbl *cmdtp, int flag,	\
 				 int argc, char *const argv[],		\