Makefile: Don't shadow actual error when compile ASL

If ASL compiler failed by any reason do not produce output C file.
Otherwise sequential run of make will shadow the actual error in ASL,
i.e.

  CC      board/intel/edison/dsdt.o
board/intel/edison/dsdt.c:1:1: error: unterminated comment
 /*

and user has to remove dsdt.c and run make in order to see the error again.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>

scripts/Makefile.lib

@@ -386,7 +386,7 @@ $(obj)/helloworld.so: $(obj)/helloworld.o arch/$(ARCH)/lib/$(EFI_CRT0) \
 quiet_cmd_acpi_c_asl= ASL     $<
 cmd_acpi_c_asl=         \
 	$(CPP) -x assembler-with-cpp -D__ASSEMBLY__ -P $(UBOOTINCLUDE) -o $<.tmp $<; \
-	iasl -p $< -tc $<.tmp $(if $(KBUILD_VERBOSE:1=), >/dev/null); \
+	iasl -p $< -tc $<.tmp $(if $(KBUILD_VERBOSE:1=), >/dev/null) && \
 	mv $(patsubst %.asl,%.hex,$<) $@
 
 $(obj)/dsdt.c:    $(src)/dsdt.asl