fix up arm bootloader configuration
    
    ksvalidate complains that "bootloader extlinux" is invalid
    ksflatten changes the bootloader option to
    "bootloader --location=mbr" we get working configuration using
    the updated option so setting the default to it.  The Jenkins
    job on pagure is failing due to invalide syntax
    
    Signed-off-by: Dennis Gilmore <dennis@ausil.us>