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>

tools/env/fw_env.c

@@ -10,6 +10,7 @@
 #define _GNU_SOURCE
 
 #include <compiler.h>
+#include <env.h>
 #include <errno.h>
 #include <env_flags.h>
 #include <fcntl.h>