revert ext3 /boot for arm images
    
    The issue has been fixed upstream with improvements to ext4 support in u-boot 2016.11,
    in Fedora I backported these fixes to uboot-tools-2016.09.01-2.fc25 and they've now
    been verified and that release is now stable so we should be good to revert the ext3
    partition workaround for F-25 GA.