fix: chain boot path points to data partition not ESP

The chain boot path (hiso_path) was set to ($root)/hiperiso which on
our layout resolves to the ESP partition. But hiperiso_x64.efi, the
chain-boot EFI binary, lives on the DATA partition (after the new
install step copies it there).

Result: chainloader ${hiso_path}/hiperiso_x64.efi resolves to
(ESP)/hiperiso/hiperiso_x64.efi which doesn't exist -> 'chain empty
failed' despite memdisk mode working.

Fix: change hiso_path to ($hiso_dev,1)/hiperiso (data partition).
Apply same fix to tftp branch where it was looking on gpt2 (ESP).

GRUB2 binary + payload rebuilt.

src/grub2/grub/grub.cfg

@@ -2628,13 +2628,14 @@ vt_device $root  hiso_dev
 if [ "$hiso_dev" = "tftp" ]; then
     set hiso_path=($root)
     for vtid in 0 1 2 3; do
-        if [ -f (hd$vtid,gpt2)/hiperiso/hiperiso.cpio ]; then
+        if [ -f (hd$vtid,gpt1)/hiperiso/hiperiso.cpio ]; then
             set hiso_iso_part=(hd$vtid,gpt1)
             set hiso_efi_part=(hd$vtid,gpt2)
             set hisodev=hd$vtid
             break
         fi
     done
+    set hiso_path=$hiso_iso_part/hiperiso
     loadfont ascii
 
     if [ -n "$hiso_efi_part" ]; then
@@ -2652,7 +2653,7 @@ else
     if [ "$prefix" = "(hiperisodisk)/grub" ]; then
         set hiso_path=(hiperisodisk)/hiperiso
     else
-        set hiso_path=($root)/hiperiso
+        set hiso_path=($hiso_dev,1)/hiperiso
     fi
 
     set hisodev=$hiso_dev