powerpc/85xx: Use kconfig fragments

Unify mpc85xx and corenet configs using fragments, to ease maintenance
and avoid the sort of drift that the previous patch fixed.

Hardware and software options are separated, with the hope that other
embedded platforms could share the software options, and to make it
easier to maintain custom/alternate configs that focus on either
hardware or software options.

Due to the previous patch, this patch should not affect the results of
any of the affected defconfigs -- only how those results are achieved.
The resulting config is more or less the union of the options that any
of the configs previously selected. No attempt was made in this (or
the previous) patch to edit out questionable options, but this patch
will make it easier to do so in future patches.

Signed-off-by: Scott Wood <scottwood@freescale.com>