WIP: live: add .treeinfo metadata

This comes up often. We should publish treeinfo metadata in to make it
easier for other tools to find what they're looking for.

This solves the ISO part of
coreos/fedora-coreos-tracker#221, but not the
"remote tree" case.

Related: coreos/fedora-coreos-tracker#221

src/cmd-buildextend-live

@@ -441,6 +441,48 @@ def generate_iso():
                       '-no-emul-boot', '-eltorito-boot',
                       os.path.join(os.path.relpath(tmpisoimages, tmpisoroot), 'cdboot.img')]
 
+    kernel_chksum = sha256sum_file(os.path.join(tmpisoimagespxe, kernel_img))
+    # this may not be the same as the initramfs we publish, based on
+    # legacy_pxe, so just always recalculate for now
+    initramfs_chksum = sha256sum_file(os.path.join(tmpisoimagespxe, "initrd.img"))
+    rootfs_chksum = sha256sum_file(os.path.join(tmpisoimagespxe, "rootfs.img"))
+
+    # Add a productmd treeinfo
+    treeinfo = f"""
+[header]
+type = productmd.treeinfo
+version = 1.1
+
+[release]
+name = {base_name}
+version = {args.build}
+
+[tree]
+arch = {basearch}
+build_timestamp = {buildmeta["coreos-assembler.build-timestamp"]}
+
+[general]
+family = {base_name}
+version = {args.build}
+name = {base_name} {args.build}
+arch = {basearch}
+timestamp = {buildmeta["coreos-assembler.build-timestamp"]}
+variant = CoreOS
+
+[checksums]
+images/pxeboot/initrd.img = sha256:{kernel_chksum}
+images/pxeboot/rootfs.img = sha256:{rootfs_chksum}
+images/pxeboot/vmlinuz = sha256:{initramfs_chksum}
+
+[images-{basearch}]
+initrd = images/pxeboot/initrd.img
+kernel = images/pxeboot/vmlinuz
+; this one is non-standard, though hopefully tools will learn to e.g. append it
+rootfs = images/pxeboot/rootfs.img
+"""
+    with open(os.path.join(tmpisoroot, ".treeinfo"), 'w') as f:
+        f.write(treeinfo)
+
     # For x86_64 and aarch64 UEFI booting
     if basearch in ("x86_64", "aarch64"):
         # Create the efiboot.img file. This is a fat32 formatted
@@ -544,7 +586,7 @@ def generate_iso():
     buildmeta['images'].update({
         'live-kernel': {
             'path': kernel_name,
-            'sha256': sha256sum_file(kernel_file)
+            'sha256': kernel_chksum
         },
         'live-initramfs': {
             'path': initramfs_name,