Bluetooth: Reassemble periodic advertising reports

Adds a buffer to the host to reassemble potentially fragmented
periodic advertising reports.
The buffer size is configurable through BT_PER_ADV_SYNC_BUF_SIZE.
Reports that cannot be reassembled because there is no buffer or because
the buffer is full will be dropped.

Signed-off-by: Bernhard Wimmer <bernhard.wimmer@nordicsemi.no>