Bluetooth: Host: Remove printk dependency from settings

Some modules use snprintk to format the settings keys. Unfortunately
snprintk is tied with printk which is very large for some embedded
systems.
To be able to have settings enabled without also enabling printk
support, change creation of settings key strings to use bin2hex, strlen
and strcpy instead.
A utility function to make decimal presentation of a byte value is
added as u8_to_dec in lib/os/dec.c
Add new Kconfig setting BT_SETTINGS_USE_PRINTK

Signed-off-by: Kim Sekkelund <ksek@oticon.com>