env: Move env_get() to env.h

Move env_get() 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/cmd/mvebu/bubt.c b/cmd/mvebu/bubt.c
index b4d371f..84d2d53 100644
--- a/cmd/mvebu/bubt.c
+++ b/cmd/mvebu/bubt.c
@@ -7,6 +7,7 @@
 #include <config.h>
 #include <common.h>
 #include <command.h>
+#include <env.h>
 #include <vsprintf.h>
 #include <errno.h>
 #include <dm.h>