cleanup python-cryptography by using setup-eggs.

component/python-cryptography/buildout.cfg

 [buildout]
 extends =
-  ../libffi/buildout.cfg
   ../openssl/buildout.cfg
   ../pkgconfig/buildout.cfg
 
@@ -11,9 +10,17 @@ parts =
 recipe = zc.recipe.egg:custom
 egg = cryptography
 environment = python-cryptography-env
+setup-eggs = ${python-cryptography-prep:eggs}
 
 [python-cryptography-env]
 PATH = ${pkgconfig:location}/bin:%(PATH)s
-PKG_CONFIG_PATH = ${libffi:location}/lib/pkgconfig:${openssl:location}/lib/pkgconfig
-LD_LIBRARY_PATH = ${libffi:location}/lib:${openssl:location}/lib
-CPATH = ${libffi:location}/include:${openssl:location}/include
+PKG_CONFIG_PATH = ${openssl:location}/lib/pkgconfig
+LD_LIBRARY_PATH = ${openssl:location}/lib
+CPATH = ${openssl:location}/include
+
+[python-cryptography-prep]
+recipe = zc.recipe.egg
+eggs =
+  ${python-cffi:egg}
+  pycparser
+  six