OvmfPkg/README: HTTPS Boot: describe host-side TLS cipher suites forwarding

In QEMU commit range 4abf70a661a5..69699f3055a5 (later fixed up in QEMU
commit 4318432ccd3f), Phil implemented a QEMU facility for exposing the
host-side TLS cipher suite configuration to OVMF. The purpose is to
control the permitted ciphers in the guest's UEFI HTTPS boot. This
complements the forwarding of the host-side crypto policy from the host to
the guest -- the other facet was the set of CA certificates (for which
p11-kit patches had been upstreamed, on the host side).

Mention the new command line options in "OvmfPkg/README".

Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Gary Lin <glin@suse.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2852


Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Gary Lin <glin@suse.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200922091827.12617-1-lersek@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>