espsecure: Run AES-XTS encryption in O(n) instead of O(n^2) time complexity (ESPTOOL-523)

[Emill]

Description of change

The espsecure tool is currently very slow at encrypting large files with AES-XTS. The code currently runs in $O(n^2)$ time complexity, which is very bad.

Some timing I did on my computer:

Size	Time
1 MByte	0.514s
2 MByte	1.973s
4 MByte	8.806s
8 MByte	37.318s

With this patch, these are the new results:

Size	Time
1 MByte	0.146s
2 MByte	0.228s
4 MByte	0.391s
8 MByte	0.572s

I have run the esptool.py automated integration tests with this change. The results were:

.......................
----------------------------------------------------------------------
Ran 23 tests in 2.970s

OK

[radimkarnis]

Hello @Emill,
thank you for contributing. This is a great improvement! Thank you for optimizing this.

I will put this PR through our internal review queue and merge. I also want to ask you to rebase on top of master, you will get a contributor credit (doesn't happen if I rebase for you).

[Emill]

Ok, rebased!

[dobairoland]

Thank you for the improvement!