Adding support to variable size Ethernet addresses to the xip command

The xip command expects and enforces that Ethernet addresses are 6 bytes long. While there's no immediate demand to support different sizes, Ethernet addresses can have different sizes. The Ethernet principal in the XIA kernel already handles variable-size Ethernet addresses, so if one patches the xip command to do the same, the code will be ready for any address that is not 6 bytes long.

The key function to change is `form_ether_xid()` in `xip/xipether.c`. Once `form_ether_xid()` and its callers have been patched, one must drop the constant `ETHER_ADDR_SIZE` throughout the code, let the kernel identify when the address is of the wrong size, and, when an error related to a wrongly-sized address happens, friendly report it to the user.