boot: Fix ASN.1 for mbedtls >= 3.1

In Mbed TLS 3.1, the private fields in the ASN.1 structure were made private.
This breaks code that accesses these private macros.

Fix this by changing the ASN.1 specific code to use a new field accessor
`ASN1_CONTEXT_MEMBER` that will be conditionally defined based on the version of
Mbed TLS that is present.

Signed-off-by: David Brown <david.brown@linaro.org>