ipq806x: add support for zImage kernel

ipq806x: add support for zImage kernel

This change enable zImage+appended dtb support in ipq806x kernel
options. The zImage will now be generated as part of the kernel
binaries. Platforms which do not have DT support enabled in U-boot
can now make use of it by generating zImage files and appending dtb
to it.

It is not used yet but it is done as a stepping stone for early IPQ806x
platforms, which did not include DT support in U-boot.

Signed-off-by: Mathieu Olivari <mathieu@codeaurora.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45662 3c298f89-4303-0410-b956-a3cf2f4a3e73