secure boot: Ensure mbedTLS enables ECDSA if signatures are checked in app

secure boot: Ensure mbedTLS enables ECDSA if signatures are checked in app

and all ECDSA to be disabled if secure boot is not enabled

Previously if ECDSA disabled in config then secure_boot_signatures.c would
fail to build (whether or not secure boot was enabled).

To avoid breaking apps that might be using the signature scheme with custom OTA
without enabling secure boot signatures in config, this change just disables
this functionality if unavailable in mbedTLS config.

Possible fix for root cause of https://github.com/espressif/esp-idf/pull/3703