Create transceiver address type

Motivated by the fact that we might want to support hardware addresses
longer than 16-bit one day. This patch makes it easier to change the
size of addresses.
Reformat sys/net/…/mac.c to coding conventions
Reformat some shell command files to coding conventions
Add getter and setter of long addresses to transceiver
Implement net_if module
Add shell commands for interface configuration
Add test cases for net_if module