build: imx: Fix 'u-boot.imx' build without full OBJTREE reference

When calling 'make u-boot.imx' the build were failing as it were
expecting the full path for the file; this regression has been
included by commit 71a988aa (imximage.cfg: run files through C
preprocessor).

The direct references for u-boot.imx were replaced by $(obj) as
config.mk handles the proper setting of it making it set to $(OBJTREE)
when required.

The build has been test using:

 - ./MAKEALL -s mx5 -s mx6
 - make u-boot.imx
 - make O=/tmp/build

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>