powerpc/configs: Rename foo_basic_defconfig to foo_base.config

powerpc/configs: Rename foo_basic_defconfig to foo_base.config

We have several "defconfigs" that are not actually full defconfigs
they are just a base set of options which are then merged with other
fragments to produce a working defconfig.

The most obvious example is corenet_basic_defconfig which only
contains one symbol CONFIG_CORENET_GENERIC=y. And in fact if you build
it as a "defconfig" that one symbol ends up undefined, because its
prerequisites are missing.

There is also mpc85xx_base_defconfig which doesn't actually enable
CONFIG_PPC_85xx.

To avoid confusion, rename these config fragments to "foo_base.config"
to make it clearer that they are not full defconfigs and are instaed
just fragments that are used to generate real defconfigs.

Reported-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190528081614.26096-1-mpe@ellerman.id.au