kbuild: Selective compile of targets in scripts/

Do not build executables unless needed.
Same goes for scripts/mod/, descend only when CONFIG_MODULES are enabled.
With inputs form: Russell King <rmk+lkml@arm.linux.org.uk> and  Brian Gerst <bgerst@quark.didntduck.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Makefile

@@ -584,7 +584,7 @@ cmd_kallsyms = $(NM) -n $< | $(KALLSYMS) $(foreach x,$(CONFIG_KALLSYMS_ALL),--al
 .tmp_kallsyms1.o .tmp_kallsyms2.o .tmp_kallsyms3.o: %.o: %.S scripts FORCE
 	$(call if_changed_dep,as_o_S)
 
-.tmp_kallsyms%.S: .tmp_vmlinux%
+.tmp_kallsyms%.S: .tmp_vmlinux% $(KALLSYMS)
 	$(call cmd,kallsyms)
 
 .tmp_vmlinux1: $(vmlinux-objs) arch/$(ARCH)/kernel/vmlinux.lds.s FORCE

scripts/Makefile

@@ -2,14 +2,22 @@
 # scripts contains sources for various helper programs used throughout
 # the kernel for the build process.
 # ---------------------------------------------------------------------------
-# docproc: 	 Preprocess .tmpl file in order to generate .sgml docs
+# kallsyms:      Find all symbols in vmlinux
+# pnmttologo:    Convert pnm files to logo files
+# conmakehash:   Create chartable
 # conmakehash:	 Create arrays for initializing the kernel console tables
 
-host-progs	:= conmakehash kallsyms pnmtologo bin2c
+hostprogs-$(CONFIG_KALLSYMS)     += kallsyms
+hostprogs-$(CONFIG_LOGO)         += pnmtologo
+hostprogs-$(CONFIG_VT)           += conmakehash
+hostprogs-$(CONFIG_PROM_CONSOLE) += conmakehash
+hostprogs-$(CONFIG_IKCONFIG)     += bin2c
+
+host-progs	:= $(sort $(hostprogs-y))
 always		:= $(host-progs)
 
-subdir-$(CONFIG_MODVERSIONS)	+= genksyms
-subdir-y	+= mod
+subdir-$(CONFIG_MODVERSIONS) += genksyms
+subdir-$(CONFIG_MODULES)     += mod
 
 # Let clean descend into subdirs
 subdir-	+= basic lxdialog kconfig package