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>