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 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>

Makefile

@@ -467,8 +467,8 @@ $(obj)u-boot.img:	$(obj)u-boot.bin
 			sed -e 's/"[	 ]*$$/ for $(BOARD) board"/') \
 		-d $< $@
 
-$(OBJTREE)/u-boot.imx : $(obj)u-boot.bin $(SUBDIR_TOOLS) depend
-		$(MAKE) -C $(SRCTREE)/arch/arm/imx-common $@
+$(obj)u-boot.imx: $(obj)u-boot.bin depend
+		$(MAKE) -C $(SRCTREE)/arch/arm/imx-common $(OBJTREE)/u-boot.imx
 
 $(obj)u-boot.kwb:       $(obj)u-boot.bin
 		$(obj)tools/mkimage -n $(CONFIG_SYS_KWD_CONFIG) -T kwbimage \

arch/arm/cpu/arm926ejs/config.mk

@@ -34,6 +34,6 @@ PLATFORM_RELFLAGS += $(PF_RELFLAGS_SLB_AT)
 
 ifneq ($(CONFIG_IMX_CONFIG),)
 
-ALL-y	+= $(OBJTREE)/u-boot.imx
+ALL-y	+= $(obj)u-boot.imx
 
 endif

arch/arm/cpu/armv7/config.mk

@@ -40,5 +40,5 @@ PF_NO_UNALIGNED := $(call cc-option, -mno-unaligned-access,)
 PLATFORM_NO_UNALIGNED := $(PF_NO_UNALIGNED)
 
 ifneq ($(CONFIG_IMX_CONFIG),)
-ALL-y	+= $(OBJTREE)/u-boot.imx
+ALL-y	+= $(obj)u-boot.imx
 endif