Part of automake VI Patch from Ralf Corsepius <corsepiu@faw.uni-ulm.de>.

> Adds variables to the custom/*cfg files to specify the location of
> tools. The purpose is to remove hard-coded paths from the Makefiles.
>
> In later steps this eases moving the tools to other locations.

c/src/make/host.cfg.in

@@ -30,6 +30,9 @@ CHMOD=@CHMOD@
 SED=@SED@
 M4=@M4@
 
+# Global tools
+PACKHEX=$(PROJECT_TOOLS)/packhex
+
 INSTALL=$(PROJECT_TOOLS)/install-if-change
 INSTALL_VARIANT=$(PROJECT_TOOLS)/install-if-change -V "$(LIB_VARIANT)"