Bluetooth: Add hexadecimal conversion logging helper

This has so far been used in SMP code as the h() function. Now it has
a more generic name and is accessible by all parts of the Bluetooth
subsystem.

Change-Id: Iae4607da9c8b3e6779d9c97f84a4e486d5258ef0
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>