otasigcheck: Don't fail a signature check if data will be wiped

If the --wipe_data command is being passed to recovery, skip the
signature check since the data will be wiped after the update is
installed

Change-Id: I6641f25abd044110faaf170ab2f7982460e77bcb
Merge pull request #5 from CyanogenMod/cm-11.0

otasigcheck: Don't fail a signature check if data will be wiped