Enable multiple XTS encryption or decryption

multiple xts_encrypt() cannot be performed because the
tweak is not updated. That means that
  xts_encrypt(buffer1, tweak)
  xts_encrypt(buffer2, tweak)
is not the same as
  xts_encrypt(concat(buffer1, buffer2), tweak)

Current patch enables such functionalities by
updating the tweak as output of the encryption.
Note that the tweak is no more constant.

The very same modification is performed
on xts_decrypt()

Signed-off-by: Pascal Brand <pascal.brand@st.com>

src/headers/tomcrypt_cipher.h

@@ -884,12 +884,12 @@ int xts_start(                int  cipher,
 int xts_encrypt(
    const unsigned char *pt, unsigned long ptlen,
          unsigned char *ct,
-   const unsigned char *tweak,
+         unsigned char *tweak,
          symmetric_xts *xts);
 int xts_decrypt(
    const unsigned char *ct, unsigned long ptlen,
          unsigned char *pt,
-   const unsigned char *tweak,
+         unsigned char *tweak,
          symmetric_xts *xts);
 
 void xts_done(symmetric_xts *xts);