packages: make it possible to (re)build only Debian/RPM packages

- (re)building Debian packages does not create RPMs mirror any more.
- Building Debian packages can be suppressed by setting the
  BUILD_DEB_PACKAGES variable to zero.

* packages/rpm/module.mk:
  - make sure sources are prepared before building a deb package, i.e. add
    an explicit dependency.

* packages/deb/module.mk:
  - building Debian packages does not require RPMs mirror, adjust
    the dependencies accordingly.
  - don't build packages if BUILD_DEB_PACKAGES is zero.

Related-bug: #1390416
Change-Id: I049342e7619bd3be80bc2e67c89ad77618b67b59
Merge "packages: make it possible to (re)build only Debian/RPM packages"