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>

SVN-Revision: 45662