Commit e36af5f3153 moved sample-plugin a dedicated directory.
The build system could not find sources for sample-plugin-* targets.
This commit adds a dedicated build-data/packages directory for
plugins and moves sample-plugin.mk there.
Change-Id: I9e3ee8858580e86ff6d4f7317300df0a5e239a01
Signed-off-by: Pierre Pfister <ppfister@cisco.com>
