kbuild: Optimize include of .*.cmd

Keep track of possible targets while we add rules - Later, only
include the command lines / dependencies as needed.

E.g., when defining how to build multi-part objects listed in
$(multi-used-y) and $(multi-used-m), add $(multi-used-y,m) to 
the variable targets. At the end of Rules.make, we then include
the saved command lines for these targets (if they exist), needed
to check whether the command changed and we hence need to rebuild.

Rules.make

@@ -134,8 +134,9 @@ $(MODVERDIR)/%.ver: %.c FORCE
 	  mv -f $@.tmp $@; \
 	fi
 
-# updates .ver files but not modversions.h
-fastdep: $(addprefix $(MODVERDIR)/,$(export-objs:.o=.ver))
+targets := $(addprefix $(MODVERDIR)/,$(export-objs:.o=.ver))
+
+fastdep: $(targets)
 ifneq ($(export-objs),)
 	@mkdir -p $(TOPDIR)/.tmp_export-objs/modules/$(RELDIR)
 	@touch $(addprefix $(TOPDIR)/.tmp_export-objs/modules/$(RELDIR)/,$(export-objs:.o=.ver))
@@ -270,6 +271,8 @@ cmd_as_o_S       = $(CC) $(a_flags) -c -o $@ $<
 %.o: %.S FORCE
 	$(call if_changed_dep,as_o_S)
 
+targets += $(real-objs-y) $(real-objs-m) $(EXTRA_TARGETS) $(MAKECMDGOALS)
+
 # Build the compiled-in targets
 # ---------------------------------------------------------------------------
 
@@ -288,6 +291,8 @@ cmd_link_o_target = $(if $(strip $(obj-y)),\
 
 $(O_TARGET): $(obj-y) FORCE
 	$(call if_changed,link_o_target)
+
+targets += $(O_TARGET)
 endif # O_TARGET
 
 #
@@ -299,6 +304,8 @@ cmd_link_l_target = rm -f $@; $(AR) $(EXTRA_ARFLAGS) rcs $@ $(obj-y)
 
 $(L_TARGET): $(obj-y) FORCE
 	$(call if_changed,link_l_target)
+
+targets += $(L_TARGET)
 endif
 
 #
@@ -318,6 +325,8 @@ $(multi-used-y) : %.o: $(multi-objs-y) FORCE
 $(multi-used-m) : %.o: $(multi-objs-m) FORCE
 	$(call if_changed,link_multi)
 
+targets += $(multi-used-y) $(multi-used-m)
+
 # Compile programs on the host
 # ===========================================================================
 
@@ -346,6 +355,7 @@ cmd_host_cc__o        = $(HOSTCC) $(HOSTLDFLAGS) -o $@ $($@-objs) \
 $(host-progs-multi): %: $(host-progs-multi-objs) FORCE
 	$(call if_changed,host_cc__o)
 
+targets += $(host-progs-single) $(host-progs-multi-objs) $(host-progs-multi) 
 
 # Descending when building
 # ---------------------------------------------------------------------------
@@ -410,9 +420,14 @@ FORCE:
 #   which is saved in .<target>.o, to the current command line using
 #   the two filter-out commands)
 
-# read all saved command lines and dependencies
+# Read all saved command lines and dependencies for the $(targets) we
+# may be building above, using $(if_changed{,_dep}). As an
+# optimization, we don't need to read them if the target does not
+# exist, we will rebuild anyway in that case.
+
+targets := $(wildcard $(sort $(targets)))
+cmd_files := $(wildcard $(foreach f,$(targets),$(dir $(f)).$(notdir $(f)).cmd))
 
-cmd_files := $(wildcard .*.cmd)
 ifneq ($(cmd_files),)
   include $(cmd_files)
 endif