source-bundle: Allow installing to somewhere other than /

You can now set BUILDSTREAM_DESTDIR in the environment to cause the
results of the source-bundle build to be collected into that directory,
instead of the default of the root directory (`/`). This makes it
eaiser to collect up the results of a build cleanly.

If elements in the source bundle depend on each other to build, then
they need to be installed to `/`, or else extra care needs to be taken
to set variables such as PATH during the build.

buildstream/data/build-module.sh.in

@@ -38,4 +38,8 @@ rm -rf '{build_root}'
 # Install the module
 echo 'Installing {name}'
 
-(cd '{install_root}'; find . | cpio -umdp /)
+if [ -z "$BUILDSTREAM_DESTDIR" ]; then
+    BUILDSTREAM_DESTDIR=/
+fi
+
+(cd '{install_root}'; find . | cpio -umdp $BUILDSTREAM_DESTDIR)