Update to shim 15
    better checking for bad linker output
    flicker-free console if there's no error output
    improved http boot support
    better protocol re-installation
    dhcp proxy support
    tpm measurement even when verification is disabled
    REQUIRE_TPM build flag
    more reproducable builds
    measurement of everything verified through shim_verify()
    coverity and scan-build checker make targets
    misc cleanups
    
    Signed-off-by: Peter Jones <pjones@redhat.com>