Since /overlay/upper appeared, -b ignored -c silently (cause it was
still checking for /overlay/etc). Now, if /overlay/upper is absent,
sysupgrade -c will fail and exit verbosely.
Fix -l to consider -c (it never did).
Clean up to always use /overlay/upper/xxx instead of still checking
for /overlay/xxx.
Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>