config: Enable kexec bzImage signature verification
    
    New kexec syscall (kexec_file_load()) can perform bzimage signature
    verification.
    
    This will re-enable kexec/kdump on secureboot systems using new syscall.
    Currently kexec/kdump is disabled on secureboot systems.
    
    User space (kexec-tools) will be modifed to automatically detect that
    running system has secureboot enabled and use new syscall instead of
    old one.
    
    Signed-off-by: Vivek Goyal <vgoyal@redhat.com>