boot_serial: Fix missing response if echo command disabled

Fixes an issue whereby when an echo command is sent in serial
recovery mode, if it is disabled, there would just be no response
at all, which is invalid operation

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>

boot/boot_serial/src/boot_serial.c

@@ -1023,11 +1023,11 @@ boot_serial_input(char *buf, int len)
         }
     } else if (hdr->nh_group == MGMT_GROUP_ID_DEFAULT) {
         switch (hdr->nh_id) {
-        case NMGR_ID_ECHO:
 #ifdef MCUBOOT_BOOT_MGMT_ECHO
+        case NMGR_ID_ECHO:
             bs_echo(buf, len);
-#endif
             break;
+#endif
         case NMGR_ID_CONS_ECHO_CTRL:
             bs_rc_rsp(0);
             break;