Upstream mbedtls maintains the default configuration in
include/mbedtls/config.h. We maintain the default configuration in
configs/config-default.h, and our PSA-specific configuration in
include/mbedtls/config.h. This means, each time we update Mbed TLS, we need
to update the default configuration file (configs/config-default.h)
manually using the copy from mbedtls.