mpc85xx: Use gzip compressed kernel on HiveAP-330

mpc85xx: Use gzip compressed kernel on HiveAP-330

After commit 1e41de2f48 ("mpc85xx: convert TL-WDR4900 v1 to simpleImage")
XZ compression of zImage was enabled. This change exposed a problem with
the HiveAP-330 images, which was fixed by foregoing the compression on
the kernel altogether with commit 98089bb8ba8
("mpc85xx: Use uncompressed kernel on the HiveAP-330").

This patch adds back the gzip compression of the kernel image by
utilizing the generic OpenWRT uImage method instead of relying on
the PowerPC bootwrapper script that did it previously.

Compile-tested: p1020/hiveap-330

Tested-by: Chris Blake <chrisrblake93@gmail.com> [run-tested]
Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
[filled in even more text]