Upgrade to Tock-1.5-rc2

[gendx]

Tock is making a 1.5 release: tock/tock#1685. Let's upgrade to that.

The last upgrade of Tock was made a couple weeks ago, so apart from minor changes in tock/tock#1648 and tock/tock#1750, OpenSK shouldn't be affected.

• Tests pass
• Appropriate changes to README are included in PR

[gendx]

Tested on nRF52840-DK on (Chrome browser):

• https://webauthn.io/
• https://webauthn.me/

with the following configurations:

• ./deploy.py --board nrf52840dk --opensk
• ./deploy.py --board nrf52840dk --crypto_bench
• ./deploy.py --board nrf52840dk --opensk --panic-console. For this configuration, also tried the "cancel touch" flow on Chrome.
• ./deploy.py --board nrf52840dk --opensk --panic-console --verbose. Compiles and flashes properly, but still fails due to The --verbose mode is impractical #89.
• ./deploy.py --board nrf52840dk --opensk --panic-console --debug. Sometimes hit the console "freeze" likely caused by [Bug] Incorrect VirtualMuxAlarm implementation tock/tock#1651.