mbedtls: rename CONFIG_MBEDTLS_ZEPHYR_ENTROPY and move it

- Slightly improve the name of this kconfig adding the suffix
  _POLL in order to highlight that this adds a callback
  function used to poll data.

- Description was also updated to point out that this symbols
  might not only use the (secure) entropy driver, but also
  generic number generators, some of which are not really
  secure.

- The symbol was move from Kconfig to Kconfig.tls-generic because
  this is where MBEDTLS_ENTROPY_C is located and since
  MBEDTLS_ENTROPY_HARDWARE_ALT depends on the former (it only
  makes sense if the entropy module is also enabled), we add
  also the "depends on".

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