BoardModulePkg: Adds PCD to load UEFI Shell image

defines two PCDs, PcdShellFile and PcdShellFileDesc,
which holds the GUID and description of the UEFI shell file to be loaded.

A PCDs based solution gives flexibility to the user to load different
images, by just overriding the DSC file.

The user can load a diagnostic image or test image during
PCDBootToShellOnly or later stages.

Cc: Isaac Oram <isaac.w.oram@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Signed-off-by: Abdul Lateef Attar <AbdulLateef.Attar@amd.com>
Reviewed-by: Isaac Oram <isaac.w.oram@intel.com>