kbuild: Don't rebuild if vmlinux if nothing changed - fix

We would rebuild vmlinux if the command line to do so changed since
the last build. Unfortunately, we forgot to read the old command line,
so we always rebuilt it.

Makefile

@@ -484,6 +484,17 @@ sums:
 # FIXME Should go into a make.lib or something 
 # ---------------------------------------------------------------------------
 
+# read all saved command lines
+
+cmd_files := $(wildcard .*.cmd)
+ifneq ($(cmd_files),)
+  include $(cmd_files)
+endif
+
+# Usage: $(call if_changed_rule,foo)
+# will check if $(cmd_foo) changed, or any of the prequisites changed,
+# and if so will execute $(rule_foo)
+
 if_changed_rule = $(if $(strip $? \
 		               $(filter-out $(cmd_$(1)),$(cmd_$(@F)))\
 			       $(filter-out $(cmd_$(@F)),$(cmd_$(1)))),\