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>
diff --git a/toolbox/r.c b/toolbox/r.c
index 3b80db7..6183677 100644
--- a/toolbox/r.c
+++ b/toolbox/r.c
@@ -18,7 +18,7 @@
     return -1;
 }
 
-int r_main(int argc, char *argv[])
+int main(int argc, char *argv[])
 {
     if(argc < 2) return usage();