OvmfPkg/README: Update the network build flags

OvmfPkg/README: Update the network build flags

The following network build flags changed due to the inclusion of
NetworkPkg/Network.fdf.inc.

  HTTP_BOOT_ENABLE -> NETWORK_HTTP_BOOT_ENABLE
  TLS_ENABLE -> NETWORK_TLS_ENABLE

This commit also adds NETWORK_ALLOW_HTTP_CONNECTIONS to reflect the
change in OvmfPkg/OvmfPkg*.dsc.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1884
Signed-off-by: Gary Lin <glin@suse.com>
Message-Id: <20190610065509.19573-1-glin@suse.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>