Change Store AWS IoT Private Key and Certificate in SPI Flash avoiding device-specific compilations

[s-hadinger]

Description:

Following discussion on #5860, the AWS IoT credentials (~800 bytes) are now moved from source code to SPI Flash in unused location in bank FF.

This means that you will not need to compile device-specific firmwares. The same AWS IoT enabled firmware can be used on multiple devices, and the credentials (Private Key and Certificate) can be loaded with the new TLSKey command.

Added base64-1.1.1 Arduino library, used only when AWS IoT is enabled.

Code impact: +500 bytes when AWS IoT is enabled. No impact on standard firmware.

Related issue (if applicable): fixes #5860

Checklist:

• The pull request is done against the latest dev branch
• Only relevant files were touched
• Only one feature/fix was added per PR.
• The code change is tested and works on core 2.3.0, 2.4.2 and 2.5.2
  only 2.4.2 and 2.5.2
• The code change pass travis tests. Your PR cannot be merged unless tests pass
• I accept the CLA.

[s-hadinger]

Note: I will change the wiki page after the PR is merged.

[arendst]

Thx for the implementation.

As it stands it will only work with 1M devices; the location has to move with other flash sizes.

I will refactor with correct offset based on used linker script (as I do with OTA too) and move the code also to settings.ino as there the flash routines are currently stored.

[s-hadinger]

Thanks Theo. I didn’t think of 4M devices. Appreciated.