Force rebuilding of src/drivers between hostapd and wpa_supplicant

This is a (hopefully) temporary workaround to allow the same source code
tree to be used for building hostapd and wpa_supplicant without having
to manually force recompilation of some files. Currently, some of the
driver wrapper files need to be built separately for hostapd and
wpa_supplicant (#ifdef's in the files based on AP functionality).

This is somewhat racy as far as parallel make execution is concerned,
i.e., it may be necessary to run "make -j#" twice (plain "make" works
fine. Since this is supposed to be a temporary workaround, there is not
much point in trying to fix this with any more complex make processing.

src/drivers/.gitignore

@@ -0,0 +1,2 @@
+build.wpa_supplicant
+build.hostapd

src/drivers/Makefile

@@ -4,6 +4,7 @@ all:
 clean:
 	for d in $(SUBDIRS); do make -C $$d clean; done
 	rm -f *~ *.o *.d
+	rm -f build.wpa_supplicant build.hostapd
 
 install:
 	@echo Nothing to be made.