build: add KERNEL_ENTRY and sort DEFAULT_DEVICE_VARS

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>