OvmfPkg: enable the generic network stack by default

OvmfPkg: enable the generic network stack by default

DHCP, PXE, and StdLib socket apps are enabled in OVMF by the sum of:
(a) a UEFI NIC driver,
(b) the generic network stack.

The only choice for (a) used to be the proprietary Intel E1000 driver,
which is cumbersome to obtain and enable.

The iPXE UEFI NIC drivers packaged with qemu-1.5 cover (a) for each NIC
type supported by qemu, and are easy to obtain & configure, even for
earlier qemu versions. Therefore enable (b) per default as well.

This doesn't take up much space; the binaries (b) adds to the firmware
don't seem to need -D FD_SIZE_2MB.

Intel's e1000 driver remains an option, requested by the -D E1000_ENABLE
build flag.

Contributed-under: TianoCore Contribution Agreement 1.0

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@14366 6f19259b-4bc3-4df7-8a09-765794883524