powerpc/mpc85xx: Set default CONFIG_BUILD_TARGET

Final U-Boot binary for mpc85xx boards which use SPL and are not PBL-based
based is u-boot-with-spl.bin. PBL is not used only on boards with e500v1
and e500v2 cores. Apparently CONFIG_E500 is set not only for e500 cores,
but also for all other mpc85xx cores e500mc, e5500 and e5600. So do not use
CONFIG_E500 and instead filter new cores with PBL based bootrom.

Signed-off-by: Pali Rohár <pali@kernel.org>