sysroot: Support /boot on root or as seperate filesystem for syslinux and u-boot

We use a similar trick to having a `sysroot -> .` symlink on the real root
here to support both /boot on root as well as on a separate filesystem.  No
matter how it's mounted `/boot/xyz` will always refer to the file you'd
expect.

This is nicer than my previous attempts at this because there's no
configuration nor auto-detection required.

src/libostree/ostree-sysroot-deploy.c

@@ -1998,6 +1998,12 @@ prepare_new_bootloader_link (OstreeSysroot  *sysroot,
   g_assert ((current_bootversion == 0 && new_bootversion == 1) ||
             (current_bootversion == 1 && new_bootversion == 0));
 
+  /* This allows us to support both /boot on a seperate filesystem to / as well
+   * as on the same filesystem. */
+  if (TEMP_FAILURE_RETRY (symlinkat (".", sysroot->sysroot_fd, "boot/boot")) < 0)
+    if (errno != EEXIST)
+      return glnx_throw_errno_prefix (error, "symlinkat");
+
   g_autofree char *new_target = g_strdup_printf ("loader.%d", new_bootversion);
 
   /* We shouldn't actually need to replace but it's easier to reuse