OvmfPkg: add support for debug console on port 0x402

OvmfPkg: add support for debug console on port 0x402

This patch adds support for a debug console on the same port that is used
by SeaBIOS.  This makes it easier to debug OVMF, because it does not mix
debug and serial output on the same device.  It also makes it easier to
leave some of the debug messages on even in release builds.

To enable it, pass "-debugcon stdio -global isa-debugcon.iobase=0x402" to
QEMU.

The new mechanism is enabled by default, but a regular serial console can
be chosen by adding -D DEBUG_ON_SERIAL_PORT to the build options.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
[jordan.l.justen@intel.com: MAX_DEBUG_MESSAGE_LENGTH=>0x100, p=>Ptr]
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@13562 6f19259b-4bc3-4df7-8a09-765794883524