<pkg>-rsync: exclude version control files

When using rsync to import package sources (typically with
PKG_OVERRIDE_SRCDIR), it often happens that these external sources
are under version control, and contain directories like .git,
.hg, etc.

Depending on the project, these directories can become pretty large
and typically have a lot of files. Moreover, they are not necessary
in the context of building the package. Therefore, this commit adds
the --cvs-exclude option to the rsync call, saving both disk space
and sync time.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>