MdeModulePkg/CapsuleApp: Add a function used to get next DevicePath

MdeModulePkg/CapsuleApp: Add a function used to get next DevicePath

BZ:https://bugzilla.tianocore.org/show_bug.cgi?id=1482

Add a new function to support Capsule-on-Disk feature.
This function is used to get next full DevicePath from a load option.

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao Wu <hao.a.wu@intel.com>
Cc: Zhang Chao B <chao.b.zhang@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Chen A Chen <chen.a.chen@intel.com>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>