buildextend-live: add --miniso switch

This switch enables the new minimal ISO packing feature (see
coreos/coreos-installer#559).

Once a coreos-installer version with miniso support is present in RHCOS
and FCOS, we can make it unconditional and drop the switch. For now,
this will allow testing in the coreos-installer upstream CI.

src/cmd-buildextend-live

@@ -51,6 +51,8 @@ parser.add_argument("--fast", action='store_true', default=False,
                     help="Reduce compression for development (FCOS only)")
 parser.add_argument("--force", action='store_true', default=False,
                     help="Overwrite previously generated installer")
+parser.add_argument("--miniso", action='store_true', default=False,
+                    help="Enable minimal ISO packing (temporary)")
 args = parser.parse_args()
 
 # Identify the builds and target the latest build if none provided
@@ -571,15 +573,27 @@ boot
                        '-no-emul-boot']
 
     # Define inputs and outputs
-    genisoargs += ['-o', tmpisofile, tmpisoroot]
+    genisoargs_final = genisoargs + ['-o', tmpisofile, tmpisoroot]
 
-    run_verbose(genisoargs)
+    if args.miniso:
+        miniso_data = os.path.join(tmpisoroot, "miniso.dat")
+        with open(miniso_data, 'wb') as f:
+            f.truncate(16 * 1024)
+
+    run_verbose(genisoargs_final)
 
     # Add MBR, and GPT with ESP, for x86_64 BIOS/UEFI boot when ISO is
     # copied to a USB stick
     if basearch == "x86_64":
         run_verbose(['/usr/bin/isohybrid', '--uefi', tmpisofile])
 
+    if args.miniso:
+        genisoargs_minimal = genisoargs + ['-o', f'{tmpisofile}.minimal', tmpisoroot]
+        os.unlink(iso_rootfs)
+        os.unlink(miniso_data)
+        run_verbose(genisoargs_minimal)
+        if basearch == "x86_64":
+            run_verbose(['/usr/bin/isohybrid', '--uefi', f'{tmpisofile}.minimal'])
 
     # We've already created a file in the ISO with initrd_ignition_padding
     # bytes of zeroes.  Find the byte offset of that file within the ISO
@@ -654,6 +668,13 @@ boot
 
     if basearch != "s390x":
         tweak_system_area(tmpisofile)
+        if args.miniso:
+            tweak_system_area(f'{tmpisofile}.minimal')
+
+    if args.miniso:
+        # this consumes the minimal image
+        run_verbose(['coreos-installer', 'iso', 'extract', 'pack-minimal',
+                     tmpisofile, f'{tmpisofile}.minimal', "--consume"])
 
     buildmeta['images'].update({
         'live-iso': {