boards: get mac address from env and move load_sernum_ethaddr() to board init

The environment is the canonical storage location of the mac address, so
we're killing off the global data location and moving everything to
querying the env directly.

Rather than have common ppc code call a board-specific function like
load_sernum_ethaddr(), have each board call it in its own board-specific
misc_init_r() function.

The boards that get converted here are:
	- kup4k/kup4x
	- pcs440ep
	- tqm8xx

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
CC: Ben Warren <biggerbadderben@gmail.com>
CC: Stefan Roese <sr@denx.de>