CC2538DK board docs: broken links

[chrysn]

The board documentation on the CC2538DK, in the section on flashing, points the reader to TI documentation on making the bootloader enterable, ("here", "Uniflash"), but both links are currently broken, and only one has a recent (Uniflash, whereas the BootloaderBackdoor already has a now-broken redirect archived).

If the linked references are not easy to replace, I think it would be useful to give the reader an easy way to detect whether the bootloader could be entered (making the nontrivial part of the flashing section moot) or not.

(Unfortunately I don't have a board at hand; I'm trying to help a colleague through this who is just getting started with RIOT and has those boards at hand.)