syslinux: Improve compatibility with u-boot extlinux.conf

The NVidia Jetson NX comes with u-boot installed in syslinux compatible
mode.  It reads from `/boot/extlinux/extlinux.conf` to know what to boot.
It's quite picky about what format it will accept.  It crashes if
`MENU LABEL` is missing or `KERNEL` is present rather than `LINUX`.

This commit improves compatibility by conforming to what u-boot accepts.
In theory it shouldn't affect real syslinux systems because the
documentation says:

> ### LINUX
>
> You can use this, instead of using KERNEL file to boot a linux kernel
> image.

although I don't have a real syslinux system to test it on.

src/libostree/ostree-bootloader-syslinux.c

@@ -92,12 +92,13 @@ append_config_from_loader_entries (OstreeBootloaderSyslinux  *self,
       if (regenerate_default && i == 0)
         g_ptr_array_add (new_lines, g_strdup_printf ("DEFAULT %s", val));
 
-      g_ptr_array_add (new_lines, g_strdup_printf ("LABEL %s", val));
+      g_ptr_array_add (new_lines, g_strdup_printf ("\nLABEL %s", val));
+      g_ptr_array_add (new_lines, g_strdup_printf ("\tMENU LABEL %s", val));
 
       val = ostree_bootconfig_parser_get (config, "linux");
       if (!val)
         return glnx_throw (error, "No \"linux\" key in bootloader config");
-      g_ptr_array_add (new_lines, g_strdup_printf ("\tKERNEL /boot%s", val));
+      g_ptr_array_add (new_lines, g_strdup_printf ("\tLINUX /boot%s", val));
 
       val = ostree_bootconfig_parser_get (config, "initrd");
       if (val)
@@ -207,6 +208,11 @@ _ostree_bootloader_syslinux_write_config (OstreeBootloader  *bootloader,
           g_free (kernel_arg);
           kernel_arg = g_strdup (line + strlen ("\tKERNEL "));
         }
+      else if (parsing_label && g_str_has_prefix (line, "\tLINUX "))
+        {
+          g_free (kernel_arg);
+          kernel_arg = g_strdup (line + strlen ("\tLINUX "));
+        }
       else if (!parsing_label &&
                (g_str_has_prefix (line, "DEFAULT ")))
         {

tests/bootloader-entries-crosscheck.py

@@ -88,6 +88,8 @@ def validate_syslinux(sysroot):
                 syslinux_entry['title'] = v
             elif k == 'KERNEL':
                 syslinux_entry['linux'] = v
+            elif k == 'LINUX':
+                syslinux_entry['linux'] = v
             elif k == 'INITRD':
                 syslinux_entry['initrd'] = v
             elif k == 'APPEND':