board: powerpc: convert more makefiles to Kbuild style

Cc: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Kim Phillips <kim.phillips@freescale.com>
Cc: York Sun <yorksun@freescale.com>
Signed-off-by: Tom Rini <trini@ti.com>