[crypto] Expose RSA_CTX_SIZE constant

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/crypto/rsa.c

 /** RSA public-key algorithm */
 struct pubkey_algorithm rsa_algorithm = {
 	.name		= "rsa",
-	.ctxsize	= sizeof ( struct rsa_context ),
+	.ctxsize	= RSA_CTX_SIZE,
 	.init		= rsa_init,
 	.max_len	= rsa_max_len,
 	.encrypt	= rsa_encrypt,

src/include/ipxe/rsa.h

 	void *tmp;
 };
 
+/** RSA context size */
+#define RSA_CTX_SIZE sizeof ( struct rsa_context )
+
 extern struct pubkey_algorithm rsa_algorithm;
 
 #endif /* _IPXE_RSA_H */