OvmfPkg: Make SOURCE_DEBUG_ENABLE actually need to be set to TRUE
    
    Currently some tests check the value of SOURCE_DEBUG_ENABLE, and some
    tests check if it's defined or not.  Additionally, in UefiPayloadPkg as
    well as some other trees, we define it as FALSE in the .dsc file.
    
    This patch changes all of the Ovmf platforms to explicitly define it as
    FALSE by default, and changes all of the checks to test if the value is
    TRUE.
    
    Signed-off-by: Peter Jones <pjones@redhat.com>
    Message-Id: <20190920184507.909884-1-pjones@redhat.com>
    [lersek@redhat.com: drop Contributed-under line, per TianoCore BZ#1373]
    [lersek@redhat.com: replace "!= TRUE" with more idiomatic "== FALSE"]
    Cc: Andrew Fish <afish@apple.com>
    Cc: Anthony Perard <anthony.perard@citrix.com>
    Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
    Cc: Jordan Justen <jordan.l.justen@intel.com>
    Cc: Julien Grall <julien.grall@arm.com>
    Cc: Leif Lindholm <leif.lindholm@linaro.org>
    Cc: Michael Kinney <michael.d.kinney@intel.com>
    Cc: Peter Jones <pjones@redhat.com>
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>
    Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
    Acked-by: Anthony PERARD <anthony.perard@citrix.com>
    Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>