convert platform.txt when building board manager packages.

The Arduino IDE handles the tool path differently for manual installs and for
packages that have been added by using the board manager.

For the compile step it is possible to overcome that problem by redefining
two automatic variables. These redefinitions only work for manual installs.
For board manager installs they are ignored and the variable keep their
automatic values. This is an ugly trick, but it works:

runtime.tools.sdcc.path={runtime.hardware.path}/tools/sdcc
runtime.tools.STM8Tools.path={runtime.hardware.path}/tools

Apparently, this trick only works for compiler settings, not for the
upload process. The upload process is not handled by arduino-build, but
the IDE itself and behaves slightly differently.
Problem is in Arduino/arduino-core/src/processing/app/BaseNoGui.java
function createToolPreferences():
	PreferencesData.removeAllKeysWithPrefix(prefix);

This manual/automatic install shisma is now overcome by patching the
platform.txt in the process of generating the board manager packages.

Before packaging, the string "{runtime.hardware.path}/tools" is replaced by
"runtime.tools.STM8Tools.path" in all tools.stm8.* lines.

This is another ugly workaround, but again, it works.
(hopefully)

board-manager/Makefile.core

@@ -43,8 +43,12 @@ $(PLATFORM_ENTRY): $(COREFILE)
 
 $(COREFILE): | $(COREDIR)
 	echo "Generating the core archive file."
-	(cd ..;	git archive -v --prefix=./ --format=tar HEAD:$(SUBTREE))|\
-	bzip2 > $(COREFILE)
+	$(eval TMPDIR := $(shell mktemp -d))
+	cd ..; git archive -v HEAD:sduino/stm8|tar -x -C $(TMPDIR)
+	sed -i '/^tools\.stm8/ s#{runtime.hardware.path}/tools#{runtime.tools.STM8Tools.path}#' \
+		$(TMPDIR)/platform.txt
+	tar cvjf $(COREFILE) -C $(TMPDIR) .
+	rm -rf $(TMPDIR)
 
 # make a new release directory if not present
 #