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

targettest/apple2/Makefile

@@ -29,7 +29,25 @@ else
   LD := $(if $(wildcard ../../bin/ld65*),../../bin/ld65,ld65)
 endif
 
-all: hgrshow hgrtest dhgrshow
+EXELIST_apple2 = \
+        hgrshow \
+        hgrtest \
+        dhgrshow
+
+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: apple2 tests not available for" $(SYS)
+else
+# suppress the "nothing to be done for 'testcode' message
+	@echo > $(NULLDEV)
+endif
 
 disk: hgr.dsk dhgr.dsk