Bluetooth: host: Device name handling of invalid length

Add a build assert if the device name has been misconfigured. The device
name has a max length of 248. When configured as dynamic make sure that
the initial device length can fit in the dynamic max length.
This prevents us from having to handle length overflow when setting
device name in advertising data which has an 8-bit length field.

Log a warning if failing to set the device name in bt_enable.
Remove unused defines in the shell.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>