imx: mkimage: avoid stop CI when required files not exists

Introduce a new script to check whether file exists and
use that check in Makefile to avoid break CI system.

The script return 1 when the required files not exists, return 0
when files exists. The script will ignore check to u-boot-dtb.bin,
because if there is something wrong to generate u-boot-dtb.bin,
there must be some code error.

Signed-off-by: Peng Fan <peng.fan@nxp.com>