memtester: improve build system

This package uses a unusual build system. Improve the prepare stage
so that we can use the provided Makefiles and scripts instead of
generating a simple Makefile without optimisation and othe useful
compiler flags.

This solution was suggested by Michael Olbrich <m.olbrich@pengutronix.de>

Signed-off-by: Markus Niebel <Markus.Niebel@tq-group.com>
Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>
zip: fix download url

Signed-off-by: Markus Niebel <Markus.Niebel@tq-group.com>
Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>
u-boot: add config option for imx images

For i.MX SOC the U-Boot build system generates imx-images
containing the DCD structure used by the ROM loader.

This patch provides a config option and install rules for
this type of images.

Signed-off-by: Markus Niebel <Markus.Niebel@tq-group.com>
Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>