Bluetooth: Host: Add choice select whether BT RX

Change CONFIG_BT_RECV_IS_RX_THREAD into a
choice:CONFIG_BT_RECV_CONTEXT with the following options
(names can be discussed further of course):

    CONFIG_BT_RECV_BLOCKING
    CONFIG_BT_RECV_WORKQ_BT
    CONFIG_BT_RECV_WORKQ_SYS

This way users would be able to choose what to run most of
the BLE stack on, they wouldn't be forced to a single model.

We would default to CONFIG_BT_RECV_BLOCKING so that we wouldn't
need to change the system workqueue stack size by default, instead
asking users to do so if they select the CONFIG_BT_RECV_WORKQ_SYS option

Signed-off-by: Lingao Meng <menglingao@xiaomi.com>