fix makefiles to respect DTC setting

Top-level Makefile has option to select dtc binary, but it is ignored
due to bug in Makefile.lib. Fix it.

Signed-off-by: Pavel Machek <pavel@denx.de>

scripts/Makefile.lib

@@ -275,7 +275,7 @@ $(obj)/%.dtb.S: $(obj)/%.dtb
 quiet_cmd_dtc = DTC     $@
 # Modified for U-Boot
 cmd_dtc = $(CPP) $(dtc_cpp_flags) -x assembler-with-cpp -o $(dtc-tmp) $< ; \
-	dtc -O dtb -o $@ -b 0 \
+	$(DTC) -O dtb -o $@ -b 0 \
 		-i $(dir $<) $(DTC_FLAGS) \
 		-d $(depfile).dtc.tmp $(dtc-tmp) ; \
 	cat $(depfile).pre.tmp $(depfile).dtc.tmp > $(depfile)