Vlv2TbltDevicePkg/PlatformBdsLib: Add DebugAgent Console

Vlv2TbltDevicePkg/PlatformBdsLib: Add DebugAgent Console

https://bugzilla.tianocore.org/show_bug.cgi?id=276

Add the UART console produced by the DebugAgent when
SOURCE_DEBUG_ENABLE is TRUE.  Without this change, the
debugger connects but the Boot Manager and the UEFI
Shell do not have an active console because the Boot
Manager does not know about the DebugAgent UART
Console device path and the Debug Agent is using the
UART device.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: David Wei <david.wei@intel.com>
Cc: Mang Guo <mang.guo@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Michael Kinney <michael.d.kinney@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>