SHA2 renamed to SHA256, SHA4 renamed to SHA512 and functions accordingly

The SHA4 name was not clear with regards to the new SHA-3 standard. So
SHA2 and SHA4 have been renamed to better represent what they are:
SHA256 and SHA512 modules.
diff --git a/include/polarssl/error.h b/include/polarssl/error.h
index 366864c..c326154 100644
--- a/include/polarssl/error.h
+++ b/include/polarssl/error.h
@@ -68,8 +68,8 @@
  * MD4       1  0x0072-0x0072
  * MD5       1  0x0074-0x0074
  * SHA1      1  0x0076-0x0076
- * SHA2      1  0x0078-0x0078
- * SHA4      1  0x007A-0x007A
+ * SHA256    1  0x0078-0x0078
+ * SHA512    1  0x007A-0x007A
  * PBKDF2    1  0x007C-0x007C
  *
  * High-level module nr (3 bits - 0x1...-0x8...)