imximage.cfg: run files through C preprocessor

The '#' used as comments in the files cause the preprocessor
trouble, so change to /* */.

The mkimage command which uses this preprocessor output
was moved to arch/arm/imx-common/Makefile

.gitignore was updated to ignore .cfgtmp files.

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>