Add support for alternative RSA implementations

Alternative RSA implementations can be provided by defining MBEDTLS_RSA_ALT in
config.h, defining an mbedtls_rsa_context struct in a new file rsa_alt.h and
re-implementing the RSA interface specified in rsa.h.

Through the previous reworkings, the adherence to the interface is the only
implementation obligation - in particular, implementors are free to use a
different layout for the RSA context structure.

include/mbedtls/config.h

@@ -267,6 +267,7 @@
 //#define MBEDTLS_BLOWFISH_ALT
 //#define MBEDTLS_CAMELLIA_ALT
 //#define MBEDTLS_DES_ALT
+//#define MBEDTLS_RSA_ALT
 //#define MBEDTLS_XTEA_ALT
 //#define MBEDTLS_MD2_ALT
 //#define MBEDTLS_MD4_ALT

include/mbedtls/rsa.h

@@ -209,6 +209,8 @@ int mbedtls_rsa_check_params( mbedtls_mpi *N, mbedtls_mpi *P, mbedtls_mpi *Q,
  * Implementation of RSA interface
  */
 
+#if !defined(MBEDTLS_RSA_ALT)
+
 /**
  * \brief          RSA context structure
  */
@@ -252,6 +254,12 @@ typedef struct
 }
 mbedtls_rsa_context;
 
+#else
+
+#include "rsa_alt.h"
+
+#endif /* MBEDTLS_RSA_ALT */
+
 /**
  * \brief          Initialize an RSA context
  *