uboot-imx

Revert "mkconfig: start deprecating Makefile config targets"

This reverts commit 1285a2808a254f3d1a809c1a541f0c0f746e03d7 since the migration
of boards from Makefile to boards.cfg is now complete.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>