OvmfPkg: Add CSM16 and related drivers if CSM_ENABLE is set

OvmfPkg: Add CSM16 and related drivers if CSM_ENABLE is set

Note: The CSM16 binary must be copied into the tree at
OvmfPkg/Csm/Csm16/Csm16.bin in order to use CSM_ENABLE.

If CSM_ENABLE is set during the build, then the CSM16 binary
and the CSM support (Legacy BIOS) drivers will be added to
the build.

Signed-off-by: jljusten
Reviewed-by: geekboy15a
git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@12682 6f19259b-4bc3-4df7-8a09-765794883524