Tweak to cross-compilation scripts

Detail:
  Now uses environment variable RISCOS to point to root of source tree.
  This makes it easier to keep the two forks of each script in line when cloning the file for a component at a different directory depth, and also allows it to be overridden from the command line.

mk.sh

@@ -7,12 +7,13 @@ TARGET=all
 [[ "$1" =~ (all|install|debug|clean|links) ]] && TARGET=$1 && shift
 [ -z "$1" ] || BUILD=$1 && shift
 [ -n "$BUILD" ] || BUILD=ROOL/CTools
+[ -n "$RISCOS" ] || RISCOS=../../../../..
 cd $(dirname $0)
 
 # Two ways of doing the same thing
 if [ -n "$BUILDUSINGSUBSHELL" ]; then
 	HERE=`pwd`
-	cd ../../../../..
+	cd $RISCOS
 	if [ $TARGET == "install" ]; then
 		Env/$BUILD.sh "cd $HERE && make install COMPONENT=squeeze INSTTYPE=tool INSTDIR=$INSTALLDIR/Library     -I\$MAKEFILEDIR --no-print-directory"
 		Env/$BUILD.sh "cd $HERE && make install COMPONENT=squeeze INSTTYPE=app  INSTDIR=$INSTALLDIR/Tools/Tools -I\$MAKEFILEDIR --no-print-directory"
@@ -20,7 +21,7 @@ if [ -n "$BUILDUSINGSUBSHELL" ]; then
 		Env/$BUILD.sh "cd $HERE && make $TARGET COMPONENT=squeeze -I\$MAKEFILEDIR --no-print-directory"
 	fi
 else
-	pushd ../../../../.. > /dev/null
+	pushd $RISCOS > /dev/null
 	source Env/$BUILD.sh
 	popd > /dev/null
 	if [ $TARGET == "install" ]; then