Allow passing absolute paths to initrd and vmlinuz

Grub needs absolute paths to initrd and vmlinuz if we do not have
`/boot` in a boot partition, which we do not in bootc.

Add param `boot_subdir` which acts like a subdirectory in the boot
directory in case the boot partition is mounted in another directory.

Signed-off-by: Pragyan Poudyal <[email protected]>

Author: Johan-Liebert1

src/main.rs

@@ -279,6 +279,7 @@ async fn main() -> Result<()> {
                     entry,
                     &id,
                     bootdir,
+                    None,
                     entry_id.as_deref(),
                     &cmdline_refs,
                 )?;