lib_bluetooth: improve controller visibility and l2ping robustness

- Add rfkill/sysfs based helpers (rfkillunblocksysfs, listhcis, findhcisysfs)
  and btgetbdaddr() to reliably discover HCI adapters and their BD_ADDR.
- Make bt_l2ping_check() more tolerant by adding a short grace period after
  connect, retrying l2ping a couple of times, and logging each attempt, which
  avoids false FAILs when manual l2ping is already stable.

Signed-off-by: Srikanth Muppandam <smuppand@qti.qualcomm.com>

Author: smuppand