[build] Avoid always rebuilding bin/embedded.o

bin/embedded.o has a build dependency on bin/.embedded.list, which
gets generated automatically by the Makefile.  However, if the
EMBEDDED_IMAGE list is empty, bin/.embedded.list will never be
created, and so bin/embedded.o will be rebuilt every time due to a
missing dependency.

Fix by forcing bin/.embedded.list to be created even if the list is
empty.

src/Makefile.housekeeping

 #
 EMBEDDED_LIST	:= $(BIN)/.embedded.list
 ifeq ($(wildcard $(EMBEDDED_LIST)),)
-EMBEDDED_LIST_IMAGE :=
+EMBEDDED_LIST_IMAGE := <invalid>
 else
 EMBEDDED_LIST_IMAGE := $(shell cat $(EMBEDDED_LIST))
 endif
 #
 BLIB_LIST	:= $(BIN)/.blib.list
 ifeq ($(wildcard $(BLIB_LIST)),)
-BLIB_LIST_OBJS	:=
+BLIB_LIST_OBJS	:= <invalid>
 else
 BLIB_LIST_OBJS	:= $(shell cat $(BLIB_LIST))
 endif