pieeprom-2025-01-27: 2712: Add PARTITION_WALK option (latest)

pieeprom-2025-01-27: 2712: Add PARTITION_WALK option (latest)

* Walk the partition table if the requested partition is not bootable
  Previously, if the specified boot partition was not bootable the
  bootloader would stop and advance to the next BOOT_ORDER. If the
  new PARTITION_WALK option is set to 1 the bootloader will now
  check each partition in turn starting from the specified partition
  before advancing the BOOT_ORDER.
  This feature is intended for use with A/B systems to handle the case
  where autoboot.txt is missing / corrupted. This change enables
  the system to failover to the next available bootable partition.
  The autoboot.txt file is not scanned during the partition-walk
  phase i.e. there is no recursive processing of autoboot.txt files.
  This option is only supported on physical block devices
  (SD, NVMe, USB) and not RAMDISK. USB assumes a single high speed
  device, partition walks on multiple USB devices is not recommended
  and may cause timeouts.
* Improve keyboard handling in boot menu
  Try and make it more likely that we have enough time to perform key
  detection.
  Ignore mice, which were being enumerated and slowing things down.