fix Makefiles for all targets, dive into subdirs like in samples

targettest/atari/Makefile

@@ -27,8 +27,30 @@ else
   LD := $(if $(wildcard ../../bin/ld65*),../../bin/ld65,ld65)
 endif
 
-all: charmapping.xex defdev.xex displaylist.xex mem.xex multi.xex ostype.xex \
-	scrcode.com sys.xex
+EXELIST_atari = \
+        charmapping.xex \
+        defdev.xex \
+        displaylist.xex \
+        mem.xex \
+        multi.xex \
+        ostype.xex \
+        scrcode.com \
+        sys.xex
+
+ifneq ($(EXELIST_$(SYS)),)
+testcode: $(EXELIST_$(SYS))
+else
+testcode: notavailable
+endif
+
+# empty target used to skip systems that will not work with any program in this dir
+notavailable:
+ifeq ($(MAKELEVEL),0)
+	@echo "info: atari tests not available for" $(SYS)
+else
+# suppress the "nothing to be done for 'testcode' message
+	@echo > $(NULLDEV)
+endif
 
 charmapping.xex: charmapping.c
 	$(CL) -t atari -o charmapping.xex charmapping.c