fix context switch in lpm_sleep

cpu/native/lpm_cpu.c

@@ -46,11 +46,19 @@ void _native_lpm_sleep()
     retval = select(1, &_native_uart_rfds, NULL, NULL, NULL);
     DEBUG("_native_lpm_sleep: retval: %i\n", retval);
     if (retval != -1) {
-        DEBUG("\n\n\t\treturn from syscall, calling _native_handle_uart0_input\n\n");
+        /* uart ready, swap to ISR context and handle input */
+        makecontext(_native_isr_ctx, _native_handle_uart0_input, 0);
+        swapcontext((ucontext_t*)(active_thread->sp), _native_isr_ctx);
+    }
+    else if ((retval == -1) && (errno == EINTR)) {
+        /* TODO: reevaluate and merge with above branch
+         *       IF any thread except the idle thread uses lpm_set this could make sense... */
+        /* select interrupted by signal swap to ISR context and handle input */
+        DEBUG("\n\n\t\treturn from interrupted syscall, swapping context and calling _native_handle_uart0_input\n\n");
         makecontext(_native_isr_ctx, _native_handle_uart0_input, 0);
         swapcontext(_native_cur_ctx, _native_isr_ctx);
     }
-    else if (errno != EINTR) {
+    else {
         err(1, "lpm_set(): select()");
     }
 #else