samples: Bluetooth: observer: Extended Scanning on BBC Micro Bit board

Add configuration overlay file to support observer sample
with Extended Scanning on BBC Micro Bit board.

Due to slow CPU, there will be assertions, and this commit
(for now) validates build-only. And the sample may run for
a duration until it asserts.

Asserts:

- ASSERTION FAIL [start_us == (aux_start_us + 1U)]
  @ WEST_TOPDIR/zephyr/subsys/bluetooth/controller/ll_sw/
  nordic/lll/lll_scan_aux.c:359
  This will happen for small aux offset value, definitely
  for the 300 us because CPU usage latency to setup such
  auxiliary PDU reception on nRF51 is high due to slow CPU.

- ASSERTION FAIL [0]
  @ WEST_TOPDIR/zephyr/subsys/bluetooth/controller/ll_sw/
  nordic/lll/lll_scan_aux.c:592
    prepare_cb: Actual EVENT_OVERHEAD_START_US = 579
  This will happen due to CPU usage latencies scheduling
  the radio events, due to slow CPU.

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