mbedtls: kconfig: MBEDTLS_USER_CONFIG_FILE default without a prompt

This commit removes the prompt from MBEDTLS_USER_CONFIG_FILE unless
MBEDTLS_USER_CONFIG_ENABLE is true.

This fixes issues where other parts would specify a default value for
MBEDTLS_USER_CONFIG_FILE that would become stuck and not updated if
user re-configured the system using menuconfig.

Disabling the prompt ensures that only when a user specifically enables
MBEDTLS_USER_CONFIG_ENABLE and specify a custom user value in
MBEDTLS_USER_CONFIG_FILE the setting will be fixed.

Also updates the manifest with related change in the mbedtls project.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>