Rename installed kernel to match name used in boot entry
    
    The mkbls() function would write 'linux /vmlinuz-${kernelver}' into the boot
    loader entry. But the code that actually copies the file would use the original
    file name with a version suffix ('cp -aT "$i" "/boot/${i##*/}-${KERNEL_VERSION}"').
    In case of a local kernel build calling /sbin/installkernel this file name was
    e.g. 'bzImage', so we would end up with '/bzImage-${KERNEL_VERSION}', which of
    course doesn't match '/vmlinuz-*'. The script would later call 'grub2-mkrel'
    on the name taken from the boot entry which would fail because the file does not
    exist. Rename the argument to "vmlinuz", so that both parts match.
    
    Tested by doing a local kernel build with 'sudo make install' at the end.
    
    Signed-off-by: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>