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>
3 files changed