build: adapt qom/Makefile and move it to Makefile.objs

qom/ already used a separate makefile.  Convert it to use relative
paths, and make it declare both common-obj-y and user-obj-y.  This
way, the upper makefiles do not need to know that some QOM files
are compiled twice.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Makefile.objs

@@ -12,9 +12,7 @@ universal-obj-y += $(qobject-obj-y)
 
 #######################################################################
 # QOM
-include $(SRC_PATH)/qom/Makefile
-qom-obj-y = $(addprefix qom/, $(qom-y))
-qom-obj-twice-y = $(addprefix qom/, $(qom-twice-y))
+qom-obj-y = qom/
 
 universal-obj-y += $(qom-obj-y)
 
@@ -94,7 +92,7 @@ fsdev-obj-$(CONFIG_VIRTFS) += $(addprefix fsdev/, $(fsdev-nested-y))
 
 common-obj-y = $(block-obj-y) blockdev.o
 common-obj-y += $(net-obj-y)
-common-obj-y += $(qom-obj-twice-y)
+common-obj-y += qom/
 common-obj-$(CONFIG_LINUX) += $(fsdev-obj-$(CONFIG_LINUX))
 common-obj-y += readline.o console.o cursor.o
 common-obj-y += $(oslib-obj-y)
@@ -202,7 +200,7 @@ user-obj-y += cutils.o cache-utils.o
 user-obj-y += module.o
 user-obj-y += qemu-user.o
 user-obj-y += $(trace-obj-y)
-user-obj-y += $(qom-obj-twice-y)
+user-obj-y += qom/
 
 ######################################################################
 # libhw
@@ -468,5 +466,8 @@ vl.o: QEMU_CFLAGS+=$(SDL_CFLAGS)
 
 QEMU_CFLAGS+=$(GLIB_CFLAGS)
 
-nested-vars += # ...
+nested-vars += \
+	qom-obj-y \
+	user-obj-y \
+	common-obj-y
 dummy := $(call unnest-vars)

qom/Makefile

-qom-y = object.o container.o qom-qobject.o
-qom-twice-y = cpu.o

qom/Makefile.objs

+qom-obj-y = object.o container.o qom-qobject.o
+qom-obj-twice-y = cpu.o
+common-obj-y = $(qom-obj-twice-y)
+user-obj-y = $(qom-obj-twice-y)