MdeModulePkg CapsuleApp: Add directory support

MdeModulePkg CapsuleApp: Add directory support

Current CapsuleApp only supports input/output file from rootdirectory.
If the CapsuleApp and related file are put into subdirectory,
below message will be shown when running the CapsuleApp in shell.

"CapsuleApp: capsule image (Capsule image file name) is not found."

This patch is to add directory support for CapsuleApp
by using shell protocol.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Feng Tian <feng.tian@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>