cmake: make the BT_HCI_TX_STACK_SIZE's prompt conditional

The BT_HCI_TX_STACK_SIZE is carefully calculated from other
options. When those options change, e.g. from a menuconfig update, it
is important that the stack size is re-calculated, but it is not when
there is a prompt.

Therefore, make the prompt conditional such that the previously set
value is only used when it has been explicitly configured to be so.

Now users can still change the value through menuconfig and prj.conf,
by also enabling <option>_WITH_PROMPT, but when the value is
calculated by the defaults, it will continue to be calculated by
defaults instead of inheriting the intial value.

This is AFAIK a novel approach, but testing has shown that it gives
the users the behaviour they want, at the cost of some boilerplate of
course. This pattern can be applied to other options if it proves to
work as intended.

Alternatively one could remove the prompt, but then it would no longer
be possible to override the value through menuconfig.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>