cc253x: optionally push bits in p2 interrupt

See Pull Request #18

cpu/cc253x/dev/port2-intr.c

@@ -33,6 +33,11 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 extern struct cc253x_p2_handler *handlers;
 
+/* avoid referencing bits, we don't call code which use them */
+#pragma save
+#if CC_CONF_OPTIMIZE_STACK_SIZE
+#pragma exclude bits
+#endif
 void
 port_2_isr(void) __interrupt(P2INT_VECTOR)
 {
@@ -47,3 +52,4 @@ port_2_isr(void) __interrupt(P2INT_VECTOR)
     P2IF = 0;
   }
 }
+#pragma restore