zephyr: Fix usage of CONFIG_MBEDTLS_BUILTIN and ASN1

This commit fixes issue where even if Zephyr provided mbedTLS
module was used, ASN1 decoding procedures have been taken from
MCUboot own version of mbedTLS extracted sources for these functions.

To be able to do that this commit changes config file used with
mbedTLS with the one provided from Zephyr (the generic one), which
allows to select required mbedTLS features via Kconfig options
exposed by Zephyr.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>