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 71a988a (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>