1) Initialize gBS, gST, gImageHandle in UefiBootServicesTableLib.c to NULL

1) Initialize gBS, gST, gImageHandle in UefiBootServicesTableLib.c to NULL
2) Add DxeCore infix to the module local variable gBS, gST, gRT, gDS in DxeMain.c. This is to solve the name conflict with UefiBootServicesTableLib which is defined in MDE Library Spec.
3) Add in check for gBS and gST in UefiDebugLibConOut/DebugLib.c and UefiDebugLibStdErr/DebugLib.c and MdePkg/Library/DxeReportStatusCodeLib/ReportStatusCodeLib.c. If they are not initialized, the API will just return with a error message.

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@2546 6f19259b-4bc3-4df7-8a09-765794883524