scripts: bossa-flash.sh: fix variable usage

We define a variable to pickup a default for the bossa binary, however
we weren't using it.  Lets do so now.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>
diff --git a/scripts/support/bossa-flash.sh b/scripts/support/bossa-flash.sh
index 348f59b..0c1fee7 100755
--- a/scripts/support/bossa-flash.sh
+++ b/scripts/support/bossa-flash.sh
@@ -7,7 +7,7 @@
 if [ `uname` = "Linux" ]; then
     stty -F /dev/ttyACM0 raw ispeed 1200 ospeed 1200 cs8 \
 -cstopb ignpar eol 255 eof 255
-    ${BOSSAC} -R -e -w -v -b "${O}/${KERNEL_BIN_NAME}"
+    ${BOSSAC_CMD} -R -e -w -v -b "${O}/${KERNEL_BIN_NAME}"
 else
     echo "CAUTION: No flash tool for your host system found!"
 fi