env: Move env_set() to env.h

Move env_set() over to the new header file.

Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Simon Glass <sjg@chromium.org>

cmd/binop.c

@@ -2,6 +2,7 @@
 
 #include <common.h>
 #include <command.h>
+#include <env.h>
 #include <hexdump.h>
 #include <malloc.h>
 #include <mapmem.h>