powerpc/ci: Fix boot issues with corenet32_smp_defconfig for e500mc

We only have fedora image specified for kernel build for
corenet32_smp_defconfig. So remove the old-image korg-8.1.0 from
the boot matrix.

Also add conditional checks to see if old-image is actually available
for doing download artifact & boot steps.

Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>

Author: riteshharjani

.github/workflows/powerpc-kernel+qemu.yml

@@ -159,7 +159,6 @@ jobs:
             machine: e500mc
             packages: qemu-system-ppc openbios-ppc
             rootfs: ppc-rootfs.cpio.gz
-            old-image: korg-8.1.0
             new-image: fedora-42
 
           - defconfig: g5_defconfig
@@ -211,10 +210,12 @@ jobs:
     - uses: actions/download-artifact@v4
       with:
         name: ${{ matrix.defconfig }}-${{ matrix.old-image }}
+      if: matrix.old-image != ''
 
     - name: Run qemu-${{ matrix.machine }} with ${{ matrix.old-image }} build kernel
       run: ./scripts/boot/qemu-${{ matrix.machine }}
+      if: matrix.old-image != ''
 
     - name: Run qemu-${{ matrix.machine_2 }} with ${{ matrix.old-image }} build kernel
       run: ./scripts/boot/qemu-${{ matrix.machine_2 }}
-      if: matrix.machine_2 != ''
+      if: matrix.machine_2 != '' && matrix.old-image != ''