imx: Create distinct pre-processed mkimage config files

Each imx image is created by a separate sub-make and during this process
the mkimage config file is run though cpp.

The cpp output is to the same file no matter what imx image is being
created.

This means if two imx images are generated in parallel they will attempt
to independently produce the same pre-processed mkimage config file at
the same time.

Avoid the problem by making the pre-processed config file name unique
based on the imx image it will be used in.  This way each image will
create a unique config file and they won't clobber each other when run
in parallel.

This should fixed the build bug referenced in b5b0e4e3 ("imximage:
Remove failure when no IVT offset is found").

Cc: Breno Lima <breno.lima@nxp.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Trent Piepho <tpiepho@impinj.com>
Tested-by: Fabio Estevam <fabio.estevam@nxp.com>