SignedCapsulePkg: Use BaseCryptLibNull to reduce package CI time
    
    https://bugzilla.tianocore.org/show_bug.cgi?id=2315
    
    Use BaseCryptLibNull for package CI builds to reduce package
    build times.  Enabled with PYTOOL_CONTINUOUS_INTEGRATION in YAML
    files.  By default PYTOOL_CONTINUOUS_INTEGRATION is not defined,
    and the original lib mappings are preserved.
    
    Cc: Jiewen Yao <jiewen.yao@intel.com>
    Cc: Chao Zhang <chao.b.zhang@intel.com>
    Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
    Reviewed-by: Liming Gao <liming.gao@intel.com>