common: Move command functions out of common.h

Move these functions into the command.h header file which is a better fit.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/board/engicam/common/board.c b/board/engicam/common/board.c
index 0c47afe..31ff297 100644
--- a/board/engicam/common/board.c
+++ b/board/engicam/common/board.c
@@ -6,6 +6,7 @@
  */
 
 #include <common.h>
+#include <command.h>
 #include <env.h>
 #include <mmc.h>
 #include <asm/arch/sys_proto.h>