build: add KERNEL_ENTRY and sort DEFAULT_DEVICE_VARS

The KERNEL_ENTRY was missing from the DEFAULT_DEVICE_VARS.

This bug was discovered while preparing alternative images
for the mpc85xx's TP-Link WDR4900-V1, which all failed to
boot due to this:
|## Booting kernel from Legacy Image at 02000000 ...
|   Image Name:   POWERPC OpenWrt Linux-4.14.96
|   Image Type:   PowerPC Linux Kernel Image (uncompressed)
|   Data Size:    2056568 Bytes = 2 MiB
|   Load Address: 01000000
|   Entry Point:  00000000
|   Verifying Checksum ... OK

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>

include/image.mk

@@ -387,12 +387,11 @@ define Device/Init
 endef
 
 DEFAULT_DEVICE_VARS := \
-  DEVICE_NAME KERNEL KERNEL_INITRAMFS KERNEL_SIZE KERNEL_INITRAMFS_IMAGE \
-  KERNEL_LOADADDR DEVICE_DTS DEVICE_DTS_CONFIG DEVICE_DTS_DIR BOARD_NAME \
-  CMDLINE UBOOTENV_IN_UBI KERNEL_IN_UBI \
-  BLOCKSIZE PAGESIZE SUBPAGESIZE VID_HDR_OFFSET \
-  UBINIZE_OPTS UIMAGE_NAME UBINIZE_PARTS \
-  SUPPORTED_DEVICES IMAGE_METADATA
+  DEVICE_NAME KERNEL KERNEL_INITRAMFS KERNEL_INITRAMFS_IMAGE KERNEL_SIZE \
+  CMDLINE UBOOTENV_IN_UBI KERNEL_IN_UBI BLOCKSIZE PAGESIZE SUBPAGESIZE \
+  VID_HDR_OFFSET UBINIZE_OPTS UBINIZE_PARTS MKUBIFS_OPTS DEVICE_DTS \
+  DEVICE_DTS_CONFIG DEVICE_DTS_DIR BOARD_NAME UIMAGE_NAME SUPPORTED_DEVICES \
+  IMAGE_METADATA KERNEL_ENTRY KERNEL_LOADADDR
 
 define Device/ExportVar
   $(1) : $(2):=$$($(2))