Bluetooth: Controller: Add LL_ASSERT_OVERHEAD define

Add LL_ASSERT_OVERHEAD define to reuse the assertion check
related to increase in actual EVENT_OVERHEAD_START_US value.

Introduce Kconfig option to permit radio event be skipped
when increased prepare callback latencies are measured,
instead of assertion.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>