build: add rules for nesting

This adds the 'magic' rules that take care of subdirectories.
The subdirectory makefiles in the source tree are not complete; they
only define some variables (listed in nested-vars) according to the
configuration.

The magic rules descend into subdirectory makefiles and gather the
evaluated values of those variables.  The values from all subdirectories
are joined together, each prefixed with the subdirectory name, and used
by the "real" makefiles.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

rules.mak

@@ -73,3 +73,42 @@ TRACETOOL=$(PYTHON) $(SRC_PATH)/scripts/tracetool.py
 
 # will delete the target of a rule if commands exit with a nonzero exit status
 .DELETE_ON_ERROR:
+
+# magic to descend into other directories
+
+obj := .
+old-nested-dirs :=
+
+define push-var
+$(eval save-$2-$1 = $(value $1))
+$(eval $1 :=)
+endef
+
+define pop-var
+$(eval subdir-$2-$1 := $(if $(filter $2,$(save-$2-$1)),$(addprefix $2,$($1))))
+$(eval $1 = $(value save-$2-$1) $$(subdir-$2-$1))
+$(eval save-$2-$1 :=)
+endef
+
+define unnest-dir
+$(foreach var,$(nested-vars),$(call push-var,$(var),$1/))
+$(eval obj := $(obj)/$1)
+$(eval include $(SRC_PATH)/$1/Makefile.objs)
+$(eval obj := $(patsubst %/$1,%,$(obj)))
+$(foreach var,$(nested-vars),$(call pop-var,$(var),$1/))
+endef
+
+define unnest-vars-1
+$(eval nested-dirs := $(filter-out \
+    $(old-nested-dirs), \
+    $(sort $(foreach var,$(nested-vars), $(filter %/, $($(var)))))))
+$(if $(nested-dirs),
+  $(foreach dir,$(nested-dirs),$(call unnest-dir,$(patsubst %/,%,$(dir))))
+  $(eval old-nested-dirs := $(old-nested-dirs) $(nested-dirs))
+  $(call unnest-vars-1))
+endef
+
+define unnest-vars
+$(call unnest-vars-1)
+$(foreach var,$(nested-vars),$(eval $(var) := $(filter-out %/, $($(var)))))
+endef