net: l2: Add a config option to allow mismatched L3/L2 address packets

The RFC1122 section 3.3.6 says we SHOULD drop the packets if L2 address
is brodcast but L3 address is unicast, but we had seen some Wi-Fi access
points in the field not conforming to that, and DHCP offer is dropped
due to this and causes Wi-Fi connectivity issues.

As the RFC saus it's SHOULD and not a MUST, add a config option to allow
such packets, disabled by default.

Signed-off-by: Chaitanya Tata <Chaitanya.Tata@nordicsemi.no>