powerpc/corenet: Add PPC_QEMU_E500 to corenet configs

Add PPC_QEMU_E500 to corenet_base.config, which is then used to generate
corenet64_smp_defconfig and corenet32_smp_defconfig.

That then allows both those configs to build kernels that boot in qemu
using the ppce500 machine type and respectively -cpu e5500 or -cpu
e500mc.

The code that is added by PPC_QEMU_E500 just defines another machine
with a probe function that recognises qemu, so there should be no change
when booting on actual hardware supported by CORENET_GENERIC.

The increase in vmlinux size is less than 1KB.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230411102838.512859-1-mpe@ellerman.id.au

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