build system: avoid full rebuild when EXTRAVERSION changes

The last two commits allow EXTRAVERSION to track the current state
of a git repository.  The build system was unable to determine
which files were affected by changes to EXTRAVERSION and caused
a full rebuild when it changed.

Alter how the version information is passed to the code so only
a handful of files need to be rebuilt when it changes.

1 files changed, 9 insertions, 1 deletions

diff --git a/Makefile b/Makefile
index c12b7791f..30a24f68e 100644
--- a/Makefile
+++ b/Makefile
@@ -892,7 +892,7 @@ endif
 # prepare2 creates a makefile if using a separate output directory
 prepare2: prepare3 outputmakefile
 
-prepare1: prepare2 include/config/MARKER
+prepare1: prepare2 include/config/MARKER include/BB_VER.h
 ifneq ($(KBUILD_MODULES),)
         $(Q)mkdir -p $(MODVERDIR)
         $(Q)rm -f $(MODVERDIR)/*
@@ -956,6 +956,13 @@ define filechk_version.h
         )
 endef
 
+define filechk_BB_VER.h
+        (echo \#define BB_VER \"$(KERNELRELEASE)\";)
+endef
+
+include/BB_VER.h: $(srctree)/Makefile .config .kernelrelease FORCE
+        $(call filechk,BB_VER.h)
+
 # ---------------------------------------------------------------------------
 
 PHONY += depend dep
@@ -1051,6 +1058,7 @@ CLEAN_FILES +=	busybox$(EXEEXT) busybox_unstripped* busybox.links \
 MRPROPER_DIRS  += include/config include2
 MRPROPER_FILES += .config .config.old include/asm .version .old_version \
                   include/NUM_APPLETS.h \
+                  include/BB_VER.h \
                   include/common_bufsiz.h \
                   include/autoconf.h \
                   include/bbconfigopts.h \