boot: pxe_utils: Add fallback support

When configured correctly, we can detect when boot fails after the boot
process has been handed over to the kernel through the use of U-Boot's
bootcount support. In some instances, such as when we are performing
atomic updates via a system such as OSTree, it is desirable to provide a
fallback option so that we can return to a previous (hopefully working)
state.

Add a "fallback" option to the supported extlinux configuration options
that points to a label like "default" so that we can utilise this in
later commits.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

include/pxe_utils.h

@@ -62,6 +62,7 @@ struct pxe_label {
  *
  * title - the name of the menu as given by a 'menu title' line.
  * default_label - the name of the default label, if any.
+ * fallback_label - the name of the fallback label, if any.
  * bmp - the bmp file name which is displayed in background
  * timeout - time in tenths of a second to wait for a user key-press before
  *           booting the default label.
@@ -73,6 +74,7 @@ struct pxe_label {
 struct pxe_menu {
 	char *title;
 	char *default_label;
+	char *fallback_label;
 	char *bmp;
 	int timeout;
 	int prompt;