board: enable random/entropy generators if BT

BT uses PSA Crypto API to perform crypto operations and, on this
platform, these APIs are implemented through Mbed TLS. In order
to properly initialize this library, a random number generator
is required.

* If the platform supports an HW entropy generator (ex: native_sim,
  nrf), then ENTROPY_GENERATOR must be used;
* Otherwise (ex: qemu_cortex_m3) test random generator can be
  enabled.

Enabling the proper option at board Kconfig level allows for
a more compact code change instead of manually editing _all_
the samples/tests that required this fix.

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