boot: Add logic to enable booting from fallback option

The "fallback" extlinux config option allows us to set an alternative
default boot option for when it has been detected that the default is
failing. Implement the logic required to boot from this option when
desired.

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