asn1_decoder: add build options for ASN1 decoder

When building with MbedTLS, we are using MbedTLS to decode ASN1 data
for x509, pkcs7 and mscode.
Introduce _LEGACY and _MBEDTLS kconfigs for ASN1 decoder legacy and
MbedTLS implementations respectively.

Signed-off-by: Raymond Mao <raymond.mao@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
diff --git a/lib/Makefile b/lib/Makefile
index 3375577..561e0d4 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -81,7 +81,7 @@
 obj-$(CONFIG_$(SPL_)SHA512_LEGACY) += sha512.o
 
 obj-$(CONFIG_CRYPT_PW) += crypt/
-obj-$(CONFIG_$(SPL_)ASN1_DECODER) += asn1_decoder.o
+obj-$(CONFIG_$(SPL_)ASN1_DECODER_LEGACY) += asn1_decoder.o
 
 obj-$(CONFIG_$(SPL_)ZLIB) += zlib/
 obj-$(CONFIG_$(SPL_)ZSTD) += zstd/