Build the optional 'r' tool separately.

You can't use $(TARGET_BUILD_VARIANT) to change how a module
is built like this makefile used to.

Change-Id: Ie6a274cab9a77eb2376a9494e9c17aae8ba0ee0b
Signed-off-by: Olivier Fourdan <olivier.fourdan@intel.com>
Signed-off-by: Guilhem IMBERTON <guilhem.imberton@intel.com>