build: Put object files into build/ folder

Instead of building in the source tree, put most object
files into the build/ folder at the root, and put each
thing that's being built into a separate folder.

This then allows us to build hostapd and wpa_supplicant
(or other combinations) without "make clean" inbetween.

For the tests keep the objects in place for now (and to
do that, add the build rule) so that we don't have to
rewrite all of that with $(call BUILDOBJS,...) which is
just noise there.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>

hs20/client/Makefile

@@ -71,19 +71,11 @@ CFLAGS += -DEAP_TLS_OPENSSL
 OBJS += ../../src/crypto/tls_openssl_ocsp.o
 LIBS += -lssl -lcrypto
 
+_OBJS_VAR := OBJS
+include ../../src/objs.mk
 hs20-osu-client: $(OBJS)
 	$(Q)$(LDO) $(LDFLAGS) -o hs20-osu-client $(OBJS) $(LIBS)
 	@$(E) "  LD " $@
 
 clean: common-clean
-	rm -f core *~ *.o *.d
-	rm -f ../../src/utils/*.o
-	rm -f ../../src/utils/*.d
-	rm -f ../../src/common/*.o
-	rm -f ../../src/common/*.d
-	rm -f ../../src/crypto/*.o
-	rm -f ../../src/crypto/*.d
-	rm -f ../../src/wps/*.o
-	rm -f ../../src/wps/*.d
-
--include $(OBJS:%.o=%.d)
+	rm -f core *~

hs20/server/Makefile

@@ -33,14 +33,10 @@ CFLAGS += $(shell xml2-config --cflags)
 LIBS += $(shell xml2-config --libs)
 OBJS += ../../src/utils/xml_libxml2.o
 
+_OBJS_VAR := OBJS
+include ../../src/objs.mk
 hs20_spp_server: $(OBJS)
 	$(LDO) $(LDFLAGS) -o hs20_spp_server $(OBJS) $(LIBS)
 
 clean: common-clean
-	rm -f core *~ *.o *.d
-	rm -f ../../src/utils/*.o
-	rm -f ../../src/utils/*.d
-	rm -f ../../src/crypto/*.o
-	rm -f ../../src/crypto/*.d
-
--include $(OBJS:%.o=%.d)
+	rm -f core *~