OvmfPkg: enable the IPv6 support

There are several network stack drivers in MdeModulePkg or NetworkPkg.
Currently, we only use the drivers from MdeModulePkg which only provides
the IPv4 support. This commit adds the IPv6 drivers in NetworkPkg into
OVMF.

Here is the table of drivers from Laszlo.

currently included  related driver  add or replace
from MdeModulePkg   in NetworkPkg   from NetworkPkg
------------------  --------------  ---------------
SnpDxe              n/a             n/a
DpcDxe              n/a             n/a
MnpDxe              n/a             n/a
VlanConfigDxe       n/a             n/a
ArpDxe              n/a             n/a
Dhcp4Dxe            Dhcp6Dxe        add
Ip4ConfigDxe        Ip6Dxe          add
Ip4Dxe              Ip6Dxe          add
Mtftp4Dxe           Mtftp6Dxe       add
Tcp4Dxe             TcpDxe          replace
Udp4Dxe             Udp6Dxe         add
UefiPxeBcDxe        UefiPxeBcDxe    replace
IScsiDxe            IScsiDxe        replace

Since the TcpDxe, UefiPxeBcDxe, and IScsiDxe drivers in NetworkPkg also
support IPv4, we replace the ones in MdeModulePkg.

To enable the IPv6 support, build OVMF with "-D NETWORK_IP6_ENABLE".
A special case is NetworkPkg/IScsiDxe. It requires openssl. For convenience,
NetworkPkg/IScsiDxe is enabled only if both IPv6 and SecureBoot are enabled.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Gary Lin <glin@suse.com>
[lersek@redhat.com: typo fix in commit message; specil -> special]
Reviewed-by: Laszlo Ersek <lersek@redhat.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16543 6f19259b-4bc3-4df7-8a09-765794883524