mbedtls: rename tls-generic Kconfig and configuration header file

Since there's now a single Kconfig and header file for Mbed TLS it makes
more sense to name them:
- Kconfig.mbedtls (instead of Kconfig.tls-generic)
- config-mbedtls.h (instead config-tls-generic.h)

Signed-off-by: Valerio Setti <vsetti@baylibre.com>