m68knommu: switch to using -mcpu= flags for ColdFire targets

Gcc has deprecated the use of the following ColdFire cpu options:
-m5206e, -m528x, -m5307 and -m5407. In there place we should use
the equivilent -mcpu= option and setting.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>