buildextend-installer: require metal4k image for live build

In practice it's required for all release images, and we've forgotten
about this before:

    coreos/fedora-coreos-pipeline#250

This increases build time for local development, but improves the
consistency of build artifacts and reduces code complexity a bit.

src/cmd-buildextend-installer

@@ -149,17 +149,19 @@ def generate_iso():
     initramfs_img = 'initramfs.img'
 
     tmpisofile = os.path.join(tmpdir, iso_name)
-    img_metal_obj = buildmeta['images'].get('metal')
-    if img_metal_obj is None:
-        raise Exception("ISO generation requires `metal` image")
-
-    img_metal = os.path.join(builddir, img_metal_obj['path'])
-    img_metal_checksum = img_metal_obj['sha256']
 
+    img_metal_obj = buildmeta['images'].get('metal')
     img_metal4k_obj = buildmeta['images'].get('metal4k')
-    if img_metal4k_obj is not None:
+    if is_live:
+        if img_metal_obj is None or img_metal4k_obj is None:
+            raise Exception("Live image generation requires `metal` and `metal4k` images")
         img_metal4k = os.path.join(builddir, img_metal4k_obj['path'])
         img_metal4k_checksum = img_metal4k_obj['sha256']
+    else:
+        if img_metal_obj is None:
+            raise Exception("ISO image generation requires `metal` image")
+    img_metal = os.path.join(builddir, img_metal_obj['path'])
+    img_metal_checksum = img_metal_obj['sha256']
 
     # Find the directory under `/usr/lib/modules/<kver>` where the
     # kernel/initrd live. It will be the 2nd entity output by
@@ -190,15 +192,13 @@ def generate_iso():
     # Add osmet files
     if is_live:
         tmp_osmet = os.path.join(tmpinitrd_base, img_metal_obj['path'] + '.osmet')
+        tmp_osmet4k = os.path.join(tmpinitrd_base, img_metal4k_obj['path'] + '.osmet')
         print(f'Generating osmet file for 512b metal image')
         run_verbose(['/usr/lib/coreos-assembler/osmet-pack',
                      img_metal, '512', tmp_osmet, img_metal_checksum])
-        if img_metal4k_obj is not None:
-            tmp_osmet4k = os.path.join(tmpinitrd_base, img_metal4k_obj['path'] + '.osmet')
-            print(f'Generating osmet file for 4k metal image')
-            run_verbose(['/usr/lib/coreos-assembler/osmet-pack',
-                         img_metal4k, '4096', tmp_osmet4k,
-                         img_metal4k_checksum])
+        print(f'Generating osmet file for 4k metal image')
+        run_verbose(['/usr/lib/coreos-assembler/osmet-pack',
+                     img_metal4k, '4096', tmp_osmet4k, img_metal4k_checksum])
 
     # Generate root squashfs
     tmp_squashfs = None