Bluetooth: Selecting ENTROPY_GENERATOR for crypto

Bluetooth sample with controller crypto, requires sys_rand32_get that
used to be linked with Tinycrypt. The selection, within Kconfig of
Tinycrypt, that has been enabling compilation of the symbol has
been removed and thus preventing controller crypto to link.

This commit moves the selection to BT_CTLR_CRYPTO.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>