Update DebugLib to provide support for "err" command in the EFI Shell to adjust the...

Update DebugLib to provide support for "err" command in the EFI Shell to adjust the filter mask for DEBUG() messages.  The "err" command provide the ability to adjust this filter mask at a global level through an EFI Variable and at the module level through a the Debug Mask Protocol.  In order to support the degree of flexibility, the DebugLib needs to use library to abstract the get/set operations to the filter mask.

1) Define new DebugPrintErrorLevelLib that produces an API that DebugPrint() APIs in DebugLib implementations can use to retrieve the current mask for filtering DebugPrint() statements.
2) Add a BASE type implementation of the DebugPrintErrorLevelLib that directly accessed PcdDebugPrintErrorLevel to provide backward compatibility with the current EDK II DebugLib behavior.
3) Update description of the DebugPrint() API in the DebugLib to describe the new dependency on the DebugPrintErrorLevelLib that replaces the prior dependency in the PcdDebugPrintErrorLevel PCD.
4) Update the comments in BaseDebugLibNull to match (3).
5) Update BaseDebugLibSerialPort to use DebugPrintErrorLevelLib instead of the PcdDebugPrintErrorLevel PCD.
6) Update UefiDebugLinConOut to use DebugPrintErrorLevelLib instead of the PcdDebugPrintErrorLevel PCD.
7) Update UefiDebugLibStdErr to use DebugPrintErrorLevelLib instead of the PcdDebugPrintErrorLevel PCD.

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@11365 6f19259b-4bc3-4df7-8a09-765794883524