GRUB package is called BR2_TARGET_GRUB

target/iso9660/Config.in

@@ -3,7 +3,7 @@ config BR2_TARGET_ROOTFS_ISO9660
 	default n
 	depends on BR2_i386
 	select BR2_TARGET_ROOTFS_EXT2
-	select BR2_PACKAGE_GRUB
+	select BR2_TARGET_GRUB
 	help
 	  Build a bootable iso9660 image