Bluetooth: log: Rework BT_ASSERT

Rework the BT_ASSERT infrastructure with the following changes:

- Transition from LOG() macros to printk()
- Allow the BT_ASSERT() macro to map directly to stancard __ASSERT()
- Allow printing of the assert reason to be disabled
- Switch from k_oops() to k_panic() configurable

There are 2 reasons for using printk() instead of LOG():

- BT_ERR uses deferred logging by default, which is problematic with
ASSERTs
- The __ASSERT() macro in Zephyr uses printk()

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>