configs/imx6sabre: Create distinct pre-processed mkimage config files

configs/imx6sabre: Create distinct pre-processed mkimage config files

Commit 0c4bccf9e882 ("configs/imxsabre: Fix U-Boot parallel build issue")
tried to fix the parallel build issue, but the real fix was developed
later by Trent Piepho later, so add such commit to fix Buildroot
build failures on rel_imx_4.9.x_1.0.0_ga NXP branch.

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/123771053
https://gitlab.com/buildroot.org/buildroot/-/jobs/123771054
https://gitlab.com/buildroot.org/buildroot/-/jobs/123771055

Reported-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>