use SHA2 unrolled code as its a bit faster

Upstream claims its faster if combined with an optimizing compiler and
I can confirm that in some tests, so lets see how it works out in
practice.

Git-Dch: Ignore

buildlib/config.h.in

@@ -50,3 +50,6 @@
 #define APT_8_CLEANER_HEADERS
 #define APT_9_CLEANER_HEADERS
 #define APT_10_CLEANER_HEADERS
+
+/* unrolling is faster combined with an optimizing compiler */
+#define SHA2_UNROLL_TRANSFORM