env: Move set_default_vars to env.h

Move this function to the new header file and rename it so it has an env_
prefix.

Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/env.h b/include/env.h
index 8082f7a..fc3519f 100644
--- a/include/env.h
+++ b/include/env.h
@@ -187,4 +187,15 @@
  */
 void env_fix_drivers(void);
 
+/**
+ * env_set_default_vars() - reset variables to their default value
+ *
+ * This resets individual variables to their value in the default environment
+ *
+ * @nvars: Number of variables to set/reset
+ * @vars: List of variables to set/reset
+ * @flags: Flags controlling matching (H_... - see search.h)
+ */
+int env_set_default_vars(int nvars, char *const vars[], int flags);
+
 #endif