do_bootm: unify duplicate prototypes

The duplication of the do_bootm prototype has gotten out of hand,
and they're pretty much all outdated (wrt constness).  Unify them
all in command.h.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/include/command.h b/include/command.h
index 00edc85..915be57 100644
--- a/include/command.h
+++ b/include/command.h
@@ -95,6 +95,8 @@
 extern int cmd_get_data_size(char* arg, int default_size);
 #endif
 
+extern int do_bootm(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[]);
+
 #endif	/* __ASSEMBLY__ */
 
 /*