boot: pxe_utils: Add fallback support

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>