This patch (against PolarSSL 1.2.7) allows alternative crypto
implementations to be compiled, without actually defining
such implementations.

* define POLARSSL_AES_ALT to include alternative AES implementation
  from polarssl/aes_alt.h

* define POLARSSL_SHA1_ALT to include alternative SHA1 implementation
  from polarssl/sha1_alt.h

* define POLARSSL_SHA2_ALT to include alternative SHA2 implementation
  from polarssl/sha2_alt.h

* define POLARSSL_SHA4_ALT to include alternative SHA4 implementation
  from polarssl/sha4_alt.h