make sure OBJDIR doesn't match CURDIR..

Makefile

@@ -30,6 +30,10 @@ ARMTARGET?= -mcpu=cortex-m3 -mthumb -DSTROBE_SINGLE_THREAD=1
 #ARMCC?=clang-mp-9.0
 #ARMTARGET?= -nostdlib -ffreestanding -target arm-none-eabi -mcpu=cortex-m3 -mfloat-abi=soft  -mthumb
 
+.if $(.OBJDIR) == $(.CURDIR)
+.error Need to set MAKEOBJDIR.
+.endif
+
 PLATFORM != uname -s
 
 .if $(PLATFORM) == "Darwin"