arm: mvebu: Avoid generating kwbimage.cfg in the source tree

arm: mvebu: Avoid generating kwbimage.cfg in the source tree

At present some boards generate kwbimage.cfg in the source tree
during the build. This breaks buildman testing on some systems
where the source tree is read-only. Update makefile rules to
generate it in the build tree instead.

Note some other boards have the kwbimage.cfg file written in
advance, hence we need check if the file exists in the build
tree first, otherwise we fall back to one in the source tree.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>