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>

src/build.rules

@@ -4,8 +4,13 @@ all: _all
 # disable built-in rules
 .SUFFIXES:
 
+# setup some variables
 ROOTDIR := $(dir $(lastword $(MAKEFILE_LIST)))
-ROOTDIR := $(dir $(ROOTDIR:%/=%))
+ROOTDIR := $(dir $(ROOTDIR:%../src/=%))../
+BUILDDIR := $(ROOTDIR)build
+_PROJ := $(abspath $(dir $(firstword $(MAKEFILE_LIST))))
+ABSROOT := $(abspath $(ROOTDIR))
+_PROJ := $(_PROJ:$(ABSROOT)/%=%)
 
 ifndef CC
 CC=gcc
@@ -26,6 +31,9 @@ endif
 ifneq ($(CONFIG_FILE),)
 -include $(CONFIG_FILE)
 
+# export for sub-makefiles
+export CONFIG_CODE_COVERAGE
+
 .PHONY: verify_config
 verify_config:
 	@if [ ! -r $(CONFIG_FILE) ]; then \
@@ -44,6 +52,10 @@ endif
 .PHONY: _all
 _all: $(VERIFY) $(ALL) $(EXTRA_TARGETS)
 
+# continue setup
+COVSUFFIX := $(if $(CONFIG_CODE_COVERAGE),-cov,)
+PROJ := $(_PROJ)$(COVSUFFIX)
+
 Q=@
 E=echo
 ifeq ($(V), 1)
@@ -59,17 +71,21 @@ ifeq ($(Q),@)
 MAKEFLAGS += --no-print-directory
 endif
 
-ifdef CONFIG_CODE_COVERAGE
-%.o: %.c
-	@$(E) "  CC " $<
-	$(Q)cd $(dir $@); $(CC) -c -o $(notdir $@) $(CFLAGS) $(notdir $<)
-else
-%.o: %.c
+_DIRS := $(BUILDDIR)/$(PROJ)
+.PHONY: _make_dirs
+_make_dirs:
+	@mkdir -p $(_DIRS)
+
+$(BUILDDIR)/$(PROJ)/src/%.o: $(ROOTDIR)src/%.c | _make_dirs
 	$(Q)$(CC) -c -o $@ $(CFLAGS) $<
 	@$(E) "  CC " $<
-endif
+$(BUILDDIR)/$(PROJ)/%.o: %.c | _make_dirs
+	$(Q)$(CC) -c -o $@ $(CFLAGS) $<
+	@$(E) "  CC " $<
+
+BUILDOBJ = $(patsubst %,$(BUILDDIR)/$(PROJ)/%,$(patsubst $(ROOTDIR)%,%,$(1)))
 
 .PHONY: common-clean
 common-clean:
 	$(Q)$(MAKE) -C $(ROOTDIR)/src clean
-	$(Q)rm -f $(ALL)
+	$(Q)rm -rf $(ALL) $(BUILDDIR)/$(PROJ)