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>
diff --git a/board/davinci/da8xxevm/da850evm.c b/board/davinci/da8xxevm/da850evm.c
index 2d57100..d3f3819 100644
--- a/board/davinci/da8xxevm/da850evm.c
+++ b/board/davinci/da8xxevm/da850evm.c
@@ -10,6 +10,7 @@
 
 #include <common.h>
 #include <dm.h>
+#include <env.h>
 #include <environment.h>
 #include <i2c.h>
 #include <net.h>