src/genconfig.sh: Use System.map file in /lib/modules if it exists
    
    In order to be able to build the kernel module in chroots where
    /boot is not populated, we should prefer using the System.map file
    that exists with the rest of the kernel build code in /lib/modules.
    
    However, in the event it doesn't exist, we should fall back to the
    location at /boot to preserve existing behavior.