The next patch will add the .imxcfg file as a rule prerequisite, so the
target is rebuilt if it changes. Instead of duplicating it in all rules,
factor out the common parts into a imx_image_rule variable.

As the arguments are now going through an eval, any use of $ must be
escaped with another $ to become $$.

No functional change.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>