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.

library/rsa.c

@@ -464,6 +464,7 @@ cleanup:
  * Default RSA interface implementation
  */
 
+#if !defined(MBEDTLS_RSA_ALT)
 
 int mbedtls_rsa_import( mbedtls_rsa_context *ctx,
                         const mbedtls_mpi *N,
@@ -2493,6 +2494,8 @@ void mbedtls_rsa_free( mbedtls_rsa_context *ctx )
 #endif
 }
 
+#endif /* !MBEDTLS_RSA_ALT */
+
 #if defined(MBEDTLS_SELF_TEST)
 
 #include "mbedtls/sha1.h"

library/version_features.c

@@ -99,6 +99,9 @@ static const char *features[] = {
 #if defined(MBEDTLS_DES_ALT)
     "MBEDTLS_DES_ALT",
 #endif /* MBEDTLS_DES_ALT */
+#if defined(MBEDTLS_RSA_ALT)
+    "MBEDTLS_RSA_ALT",
+#endif /* MBEDTLS_RSA_ALT */
 #if defined(MBEDTLS_XTEA_ALT)
     "MBEDTLS_XTEA_ALT",
 #endif /* MBEDTLS_XTEA_ALT */