When IPv6 is enabled, the MTU is given. So users(*) sending IPv6 packets can
easily figure out what the supported maximum protocol unit is.

However, when IPv6 is disabled and a user wants to send layer 2 frames directly,
no information about the maximum PDU is available using the shell.

When 6LoWPAN is used, a user may be interested in the layer 2 PDU as well in
order to avoid layer 2 fragmentation.

This PR adds the L2-PDU info to the output of the ifconfig shell command, which
is printed regardless of the use of IPv6.

(*): Here "users" refers to human beings interacting with the shell.
Applications can get the maximum PDU of each layer more easily using
gnrc_netapi_get() with NETOPT_MAX_PACKET_SIZE instead of using a shell command.