infra: centralize rsync exclude list for VCS files

Buildroot has three places where rsync is used:
1. to copy the target skeleton
2. to copy the rootfs overlay(s)
3. to copy overridden package sources

In all of these cases, we want to exclude version control files by default.
Place 1 and 2 used an identical set of explicit --exclude options, while
place 3 used the option --cvs-exclude. This last option, however, not only
excludes version control files, but also binary files (.o, .so) and any file
or directory named 'core' (a problem for the linux kernel that has several
directories with this name). Moreover, the exact list of excluded files when
using --cvs-exclude depends on the version of rsync.

This patch creates one global variable RSYNC_VCS_EXCLUSIONS that can be used
by the various rsync commands. It excludes the version control files of
svn, git, hg, cvs and bzr.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>