twrpDigest refactor

This patch is to refactor twrpDigest using polymorphism
and inheritance to use the same call patterns for creating and
reading a digest. Now a library.
Use SHA2 from libcrypto. SHA2 is default if device has libcrypto.

Change string MD5 everywhere to use digest or Digest instead. Updated
string tags to digest. Translation will be required.

Switch out digest code into a driver class from partitionmanager.

SHA2 is better for digest creation due to decreased collision space
compared to MD5 and SHA1.

See https://en.wikipedia.org/wiki/SHA-2

Change-Id: I74b5546789990b12aa4ce2e389d25f80a3fe213f
diff --git a/twrp-functions.hpp b/twrp-functions.hpp
index 82a4c1b..1f6b9b1 100644
--- a/twrp-functions.hpp
+++ b/twrp-functions.hpp
@@ -22,6 +22,8 @@
 #include <string>
 #include <vector>
 
+#include "twrpDigest/twrpDigest.hpp"
+
 using namespace std;
 
 typedef enum