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.

Note: This PR has been modified from the upstream commit due to
a merge failure from other changes that are not being brought into
Zephyr 3.7

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
(cherry picked from commit 3f672134)