mbedtls: rename MBEDTLS_ENTROPY_ENABLED as MBEDTLS_ENTROPY_C

ENABLED suffix does not really makes much sense for a kconfig
so it should be removed. Also other MBEDTLS symbols were recently
updated accordingly.
Moreover having it named exactly the same way as in Mbed TLS
symplifies the understanding of what this kconfig is doing.

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