s390x: generate GPG keys for Ignition config protection

During `cosa buildextend-secex` a pair of GPG keys is randomly generated,
where private key becomes part of `sdboot` image, and public key becomes
part of build artifacts.

User than can encrypt his Ignition config:
```
gpg --recipient-file /path/to/ignition.gpg.pub --output /path/to/config.ign.gpg --armor --encrypt /path/to/config.ign
```

And attach it to `qemu-kvm` as a disk:
```
-drive if=none,id=ignition,format=raw,file=/path/to/config.ign.gpg,readonly=on \
-device virtio-blk,serial=ignition.gpg,iommu_platform=on,drive=ignition
```

src/create_disk.sh

@@ -461,6 +461,15 @@ chroot_run() {
     done
 }
 
+generate_gpgkeys() {
+    tmp_home=$(mktemp -d /tmp/gpg-XXXXXX)
+    gpg --homedir "${tmp_home}" --batch --passphrase '' --yes --quick-gen-key secex default
+    gpg --homedir "${tmp_home}" --armor --export secex > "/srv/builds/latest/s390x/ignition.gpg.pub"
+    gpg --homedir "${tmp_home}" --armor --export-secret-key secex > "/tmp/ignition.asc"
+    touch "$deploy_root/etc/ignition.asc"
+    rm -rf "${tmp_home}"
+}
+
 # Other arch-specific bootloader changes
 # shellcheck disable=SC2031
 case "$arch" in
@@ -495,13 +504,15 @@ s390x)
         # in case builder itself runs with SecureExecution
         rdcore_zipl_args+=("--secex-mode=disable")
         chroot_run /usr/lib/dracut/modules.d/50rdcore/rdcore zipl "${rdcore_zipl_args[@]}"
+    else
+        generate_gpgkeys
     fi
     ;;
 esac
 
 # enable support for GRUB password
 # shellcheck disable=SC2031
-if "$arch" != "s390x"; then
+if [ "$arch" != "s390x" ]; then
     ostree config --repo $rootfs/ostree/repo set sysroot.bls-append-except-default 'grub_users=""'
 fi
 
@@ -589,9 +600,11 @@ if [[ ${secure_execution} -eq 1 ]]; then
     if [ ! -e /dev/disk/by-id/virtio-genprotimg ]; then
         echo "Building local Secure Execution Image, running zipl and genprotimg"
         # run zipl with root hashes as kargs
+        mount -o ro,bind "/tmp/ignition.asc" "$deploy_root/etc/ignition.asc"
         rdcore_zipl_args+=("--secex-mode=enforce" "--hostkey=/dev/disk/by-id/virtio-hostkey")
         rdcore_zipl_args+=("--append-karg=rootfs.roothash=$(cat /tmp/root-roothash)")
         rdcore_zipl_args+=("--append-karg=bootfs.roothash=$(cat /tmp/boot-roothash)")
+        rdcore_zipl_args+=("--append-file=/etc/ignition.asc")
         chroot_run /usr/lib/dracut/modules.d/50rdcore/rdcore zipl "${rdcore_zipl_args[@]}"
     else
         echo "Building release Secure Execution Image, zipl and genprotimg will be run later"