I am programming the device with USB boot loader, and for now I see only way to switch to it is to erase page at 0000.0000 and reboot. But this approach does not allow to cancel firmware update. Using boot_loader library from TivaWare is not possible, as I do not have enough free RAM to move library to, as it should work from RAM.
There sould be any way to call ROM-based boot loader from software, isn't it?