ipq40xx: sysupgrade: allow flashing Linksys factory firmware

ipq40xx: sysupgrade: allow flashing Linksys factory firmware

Allow forced flashing of a factory firmware image, after checking for the
correct FIT magic header and Linksys board-specific footer. Details of the
footer are already described in scripts/linksys-image.sh.

This is convenient as it avoids using a TFTP server or OEM GUI, and allows
restoring OEM firmware or installing a "breaking" OpenWrt update (e.g DSA
migration and kernel repartition) directly from the command line.

Devices supported at this time include EA6350v3, EA8300, MR8300 and WHW01.

Reviewed-by: Robert Marko <robimarko@gmail.com>
Tested-by:  Wyatt Martin <wawowl@gmail.com> # WHW01
Tested-by:  Tony Ambardar <itugrok@yahoo.com> # EA6350v3
Signed-off-by: Tony Ambardar <itugrok@yahoo.com>
(cherry picked from commit 6fc334cbdc2b1716786768c545b761b338962b43)