loader: command_errmsg should be const

Use const char * for command_errmsg.

stand/common/bootstrap.h

@@ -36,7 +36,7 @@
 /* Commands and return values; nonzero return sets command_errmsg != NULL */
 typedef int	(bootblk_cmd_t)(int argc, char *argv[]);
 #define	COMMAND_ERRBUFSZ	(256)
-extern char	*command_errmsg;	
+extern const char *command_errmsg;
 extern char	command_errbuf[COMMAND_ERRBUFSZ];
 #define CMD_OK		0
 #define CMD_WARN	1

stand/common/commands.c

@@ -32,7 +32,7 @@ __FBSDID("$FreeBSD$");
 
 #include "bootstrap.h"
 
-char		*command_errmsg;
+const char	*command_errmsg;
 /* XXX should have procedural interface for setting, size limit? */
 char		command_errbuf[COMMAND_ERRBUFSZ];