OvmfPkg: introduce PcdCsmEnable feature flag

OvmfPkg: introduce PcdCsmEnable feature flag

In the DXE phase and later, it is possible for a module to dynamically
determine whether a CSM is enabled. An example can be seen in commit
855743f71774 ("OvmfPkg: prevent 64-bit MMIO BAR degradation if there is no
CSM", 2016-05-25).

SEC and PEI phase modules cannot check the Legacy BIOS Protocol however.
For their sake, introduce a new feature PCD that simply reflects the
CSM_ENABLE build flag.

Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Julien Grall <julien@xen.org>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1512
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20200129214412.2361-11-lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>