dfu:core: Export dfu_{get|free}_buf functions

Define the dfu_get_buf() and dfu_free_buf() as global functions.
They are necessary for zero copy buffer management, when DFU backend is
used for storing data.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
diff --git a/drivers/dfu/dfu.c b/drivers/dfu/dfu.c
index 4ec330c..4a8804e 100644
--- a/drivers/dfu/dfu.c
+++ b/drivers/dfu/dfu.c
@@ -67,14 +67,14 @@
 static unsigned char *dfu_buf;
 static unsigned long dfu_buf_size = CONFIG_SYS_DFU_DATA_BUF_SIZE;
 
-static unsigned char *dfu_free_buf(void)
+unsigned char *dfu_free_buf(void)
 {
 	free(dfu_buf);
 	dfu_buf = NULL;
 	return dfu_buf;
 }
 
-static unsigned char *dfu_get_buf(void)
+unsigned char *dfu_get_buf(void)
 {
 	char *s;