It seems that there are parts missing from the nRF5x SDK v15 if attempting to use the microecc backend, the first is that it expects NRF_CRYPTO_HASH_ENABLED to be defined but this is only defined in CC310, MBEDTLS, NRF_SW and oberon backends - not the micro-ECC bankend.
Secondly, g_nrf_crypto_hash_sha256_info is not defined if using micro-ecc, if "#define NRF_CRYPTO_HASH_SHA256_ENABLED 1" if manually added to the sdk_config.h file (which again, is not documented) then it still fails to compile due to nrf_crypto_backend_hash_sha256_context_t being an undefined type.