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/amlogic/p200/p200.c b/board/amlogic/p200/p200.c
index 62f0f4c..7f33ce0 100644
--- a/board/amlogic/p200/p200.c
+++ b/board/amlogic/p200/p200.c
@@ -5,6 +5,7 @@
 
 #include <common.h>
 #include <dm.h>
+#include <env.h>
 #include <environment.h>
 #include <asm/io.h>
 #include <asm/arch/gx.h>