Commit bb46b635df48 changed its partition scheme, but sysupgrade image
validation still uses the old format. This commit fixes it so that
force flag is not needed for sysupgrade.
Fixes: bb46b635df48 ("ar71xx: move Arduino Yun to generic building code")
Signed-off-by: Sungbo Eo <mans0n@gorani.run>
(cherry picked from commit 58dc1d0637425cfe023192466e6212009332b677)
f17174e843