Fix memtest hook: grub.cfg/live.cfg missing during binary hooks is expected

lb binary_grub-efi and lb binary_syslinux create these files from templates
that already have memtest entries hardcoded. The hook should not fail when
the files don't exist yet — validate_iso_memtest() checks the final ISO.
Only the binary files (x64.bin, x64.efi) are required here.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/builder/config/hooks/normal/9100-memtest.hook.binary

@@ -26,6 +26,14 @@ fail_or_warn() {
     return 0
 }
 
+# grub.cfg and live.cfg may not exist yet when binary hooks run — live-build
+# creates them after this hook (lb binary_grub-efi / lb binary_syslinux).
+# The template already has memtest entries hardcoded, so a missing config file
+# here is not an error; validate_iso_memtest() checks the final ISO instead.
+warn_only() {
+    log "WARNING: $1"
+}
+
 copy_memtest_file() {
     src="$1"
     dst_name="${2:-$(basename "$src")}"
@@ -135,7 +143,7 @@ ensure_memtest_binaries() {
 
 ensure_grub_entry() {
     [ -f "$GRUB_CFG" ] || {
-        fail_or_warn "missing ${GRUB_CFG}"
+        warn_only "missing ${GRUB_CFG} (will be created by lb binary_grub-efi from template)"
         return 0
     }
 
@@ -161,7 +169,7 @@ EOF
 
 ensure_isolinux_entry() {
     [ -f "$ISOLINUX_CFG" ] || {
-        fail_or_warn "missing ${ISOLINUX_CFG}"
+        warn_only "missing ${ISOLINUX_CFG} (will be created by lb binary_syslinux from template)"
         return 0
     }