ipq806x: add support for non-dt enabled ap148 bootloader

Certain AP148 platforms (and derivative) use bootloaders which did not
have DT enabled.
In order to support these old platforms, we'll now make the following
modifications:
*explicitely add the memory node in the AP148 DT: this used to be added
 by new u-boot through a run-time patch mechanism. We'll now add it
 explicitely so it works on boots which don't support that feature. New
 boots will have the node twice, the second one will be ignored.
*add the zImage generation next to the FIT image for AP148.

Other platforms using non-DT enabled bootloaders may want to leverage
this zImage code to generate their own firmare as well.

Signed-off-by: Mathieu Olivari <mathieu@codeaurora.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46555 3c298f89-4303-0410-b956-a3cf2f4a3e73