env: Move get/set_default_env() to env.h

Move these functions to the new header file and rename set_default_env()
to env_set_default() so that it has a consistent env_ prefix.

Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/env/sata.c b/env/sata.c
index a2ff5c6..a5364dd 100644
--- a/env/sata.c
+++ b/env/sata.c
@@ -8,6 +8,7 @@
 #include <common.h>
 
 #include <command.h>
+#include <env.h>
 #include <environment.h>
 #include <linux/stddef.h>
 #include <errno.h>
@@ -106,7 +107,7 @@
 	}
 
 	if (read_env(sata, CONFIG_ENV_SIZE, CONFIG_ENV_OFFSET, buf)) {
-		set_default_env(NULL, 0);
+		env_set_default(NULL, 0);
 		return -EIO;
 	}