[v2.1.x] lib: updatehub: Improve download on slow networks and security fix

[nandojve]

Describe the bug
Fix download on slow networks (2G modems).

To Reproduce
Use of UpdateHub on wireless networks with slow signal and re-transmissions.

Expected behavior
Improve user experience and apply missing security fix.

Zephyr 2.1

[github-actions]

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