Simplify linking/building objects in subdirectories

New-style Makefiles have a nice way of declaring objects
which need to be built either built-in or as modules:

        obj-$(CONFIG_EEPRO100) += eepro100.o

However, handling objects in subdirectories, which need to be
built and linked is not as nice:

        subdir-$(CONFIG_E100) += e100

        ifeq ($(CONFIG_E100),y)
          obj-y += e100/built-in.o
        endif

This means we descend into the subdirectory when building
vmlinux / modules, depending on CONFIG_XXX. When we are building
vmlinux we also need to link whatever has been built in the
subdirectory, so we add it to $(obj-y) at the appropriate place
(link order is important).

Now, the extension below allows to rewrite the second case into

         obj-$(CONFIG_E100) += e100/

which looks much nicer ;-) Existing behavior is not changed, and the
only prerequisite to using the extension above is that the O_TARGET in
the subdir is named "built-in.o".

Rules.make

@@ -43,6 +43,27 @@ obj-m := $(filter-out $(obj-y),$(obj-m))
 #
 first_rule: all_targets
 
+# Handle objects in subdirs
+# ---------------------------------------------------------------------------
+# o if we encounter foo/ in $(obj-y), replace it by foo/built-in.o
+#   and add the directory to the list of dirs to descend into: $(subdir-y)
+# o if we encounter foo/ in $(obj-m), remove it from $(obj-m) 
+#   and add the directory to the list of dirs to descend into: $(subdir-m)
+
+__subdir-y	:= $(patsubst %/,%,$(filter %/, $(obj-y)))
+subdir-y	+= $(__subdir-y)
+__subdir-m	:= $(patsubst %/,%,$(filter %/, $(obj-m)))
+subdir-m	+= $(__subdir-m)
+__subdir-n	:= $(patsubst %/,%,$(filter %/, $(obj-n)))
+subdir-n	+= $(__subdir-n)
+__subdir-	:= $(patsubst %/,%,$(filter %/, $(obj-)))
+subdir-		+= $(__subdir-)
+obj-y		:= $(patsubst %/, %/built-in.o, $(obj-y))
+obj-m		:= $(filter-out %/, $(obj-m))
+
+# If a dir is selected in $(subdir-y) and also mentioned in $(mod-subdirs),
+# add it to $(subdir-m)
+
 both-m          := $(filter $(mod-subdirs), $(subdir-y))
 SUB_DIRS	:= $(subdir-y)
 MOD_SUB_DIRS	:= $(sort $(subdir-m) $(both-m))