net: samples: Add CONFIG_NET_L2_BLUETOOTH_ZEP1656

This enables these samples to interoperate with Linux 6LoWPAN over
Bluetooth implementation while that is being fixed, in the long term
we might remove it from the samples once Linux is fixed.

Jira: ZEP-1656

Change-Id: I8ca9fe5a27f43cebc75b6fe5a436a5e8fcee26d5
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>