Merge patch series to add a "fallback" keyword to extlinux.conf parsing

This series from Martyn Welch <martyn.welch@collabora.com> adds the
ability to have a "fallback" option in extlinux.conf parsing, which can
be in turn used in A/B style update mechanisms.

Link: https://lore.kernel.org/u-boot/20241009131548.929439-1-martyn.welch@collabora.com/