lib/rypto: Adapt rsa_helper to MbedTLS

Previous patch has introduced MbedTLS porting layer for RSA helper,
here to adjust the makefile accordingly.

Signed-off-by: Raymond Mao <raymond.mao@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
diff --git a/lib/crypto/Makefile b/lib/crypto/Makefile
index 3caa45d..72b413d 100644
--- a/lib/crypto/Makefile
+++ b/lib/crypto/Makefile
@@ -13,7 +13,7 @@
 #
 # RSA public key parser
 #
-obj-$(CONFIG_$(SPL_)RSA_PUBLIC_KEY_PARSER) += rsa_public_key.o
+obj-$(CONFIG_$(SPL_)RSA_PUBLIC_KEY_PARSER_LEGACY) += rsa_public_key.o
 rsa_public_key-y := \
 	rsapubkey.asn1.o \
 	rsa_helper.o