zynq: reworked image creation process

zynq: reworked image creation process

After commit a1248da (zynq: convert to new image build code),
all boards, became unbootable, entering into a boot loop.

Replace the compressed kernel zImage by an uncompressed Image,
which is compressed after with gzip. Also, enable the creation
of an initramfs image by default.

Change images' filename to match the compatible string from the
respective .dts file (<vendor>_<model>). Also, use the shared
mkits.sh script and remove the target specific one.

Fixes: FS#1204

Run-tested: ZedBoard

Signed-off-by: Luis Araneda <luaraneda@gmail.com>