Put dependency files in proper subdir

This seems to resolve subtle breakages of our build system:

Dependency files generated for targets like 'dir/foo.o' were saved as
'foo.d'. Now, if there was also a target 'foo.o', one of the dependency
file was overwritten. Concrete example: libhw*/macio.o vs.
libhw*/ide/macio.o. And this often left a segfaulting build result
behind when changing the "wrong" data structures".

Fix it by generating proper 'dir/foo.d'.
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Acked-by: NPaolo Bonzini <pbonzini@redhat.com>
Acked-by: NJuan Quintela <quintela@redhat.com>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

rules.mak

@@ -12,7 +12,7 @@ MAKEFLAGS += -rR
 %.mak:
 
 # Flags for dependency generation
-QEMU_DGFLAGS += -MMD -MP -MT $@
+QEMU_DGFLAGS += -MMD -MP -MT $@ -MF $(*D)/$(*F).d
 
 %.o: %.c
 	$(call quiet-command,$(CC) $(QEMU_CFLAGS) $(QEMU_DGFLAGS) $(CFLAGS) -c -o $@ $<,"  CC    $(TARGET_DIR)$@")