New function mbedtls_rsa_get_bitlen

Add a new function mbedtls_rsa_get_bitlen which returns the RSA key
size, i.e. the bit size of the modulus. In the pk module, call
mbedtls_rsa_get_bitlen instead of mbedtls_rsa_get_len, which gave the
wrong result for key sizes that are not a multiple of 8.

This commit adds one non-regression test in the pk suite. More tests
are needed for RSA key sizes that are a multiple of 8.

This commit does not address RSA alternative implementations, which
only provide an interface that return the modulus size in bytes.

include/mbedtls/rsa.h

@@ -403,6 +403,16 @@ void mbedtls_rsa_set_padding( mbedtls_rsa_context *ctx, int padding,
  */
 size_t mbedtls_rsa_get_len( const mbedtls_rsa_context *ctx );
 
+/**
+ * \brief          This function retrieves the length of the RSA modulus in bits.
+ *
+ * \param ctx      The initialized RSA context.
+ *
+ * \return         The length of the RSA modulus in bits.
+ *
+ */
+size_t mbedtls_rsa_get_bitlen( const mbedtls_rsa_context *ctx );
+
 /**
  * \brief          This function generates an RSA keypair.
  *