x86: increase bios boot partition to 1 MiB (#11741)

Author: hex-ci

target/linux/x86/image/Makefile

@@ -53,7 +53,7 @@ define Build/combined
 		$@ \
 		$(CONFIG_TARGET_KERNEL_PARTSIZE) $@.boot \
 		$(CONFIG_TARGET_ROOTFS_PARTSIZE) $(IMAGE_ROOTFS) \
-		256
+		1024
 endef
 
 define Build/grub-config