build-kernel: release .rpm alongside .deb + .pkg.tar.zst (#22)

Now that ps5-linux-image's --distro all also produces an .rpm (paired
image PR), pick it up in the artifact upload, checksum list and
release files: block. Also add rpm as an explicit format choice so
someone can dispatch an rpm-only kernel build.

The sha256sum pipeline gets a 2>/dev/null so it doesn't complain
when an older ps5-linux-image checkout is used that only writes
deb + pkg.tar.zst.

Co-authored-by: Your Name <you@example.com>

.github/workflows/build-kernel.yml

@@ -8,7 +8,7 @@ on:
         description: 'Package format'
         default: 'all'
         type: choice
-        options: [deb, arch, all]
+        options: [deb, arch, rpm, all]
       patches_ref:
         description: 'Patches branch/tag/SHA (empty = default)'
         default: ''
@@ -61,6 +61,7 @@ jobs:
           path: |
             image/linux-bin/*.deb
             image/linux-bin/*.pkg.tar.zst
+            image/linux-bin/*.rpm
           retention-days: 7
 
       - name: Compute checksums
@@ -68,7 +69,7 @@ jobs:
         run: |
           {
             echo 'sums<<SHAEOF'
-            sha256sum image/linux-bin/*.deb image/linux-bin/*.pkg.tar.zst | sed 's|image/linux-bin/||'
+            sha256sum image/linux-bin/*.deb image/linux-bin/*.pkg.tar.zst image/linux-bin/*.rpm 2>/dev/null | sed 's|image/linux-bin/||'
             echo 'SHAEOF'
           } >> "$GITHUB_OUTPUT"
 
@@ -90,4 +91,5 @@ jobs:
           files: |
             image/linux-bin/*.deb
             image/linux-bin/*.pkg.tar.zst
+            image/linux-bin/*.rpm
           make_latest: true