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>

env/ext4.c

@@ -21,6 +21,7 @@
 #include <common.h>
 
 #include <command.h>
+#include <env.h>
 #include <environment.h>
 #include <linux/stddef.h>
 #include <malloc.h>
@@ -127,7 +128,7 @@ static int env_ext4_load(void)
 	return env_import(buf, 1);
 
 err_env_relocate:
-	set_default_env(NULL, 0);
+	env_set_default(NULL, 0);
 
 	return -EIO;
 }