Calculate SHA256 over firmware image uploaded via OTA

[aentinger]

This PR calculate the SHA256 checksum over the firmware stored in the flash of the MCU. The checksum is then stored
as ASCII encoded hexadecimal number within a hidden character string property OTA_SHA256.

Note: As we don't know the length per-se we read chunks of the flash until we detect one containing only 0xFF (= flash erased). This only works for firmware updated via OTA and second stage bootloaders (SxU family) because only those erase the complete flash before performing an update.

Since the SHA256 firmware image is only required for the cloud servers to perform a version check after the OTA update this is a acceptable trade off. The bootloader is excluded from the calculation and occupies flash address range 0 to 0x2000, total flash size of 0x40000 bytes (256 kByte).

FIY @eclipse1985

[codecov-commenter]

Codecov Report

Merging #174 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #174   +/-   ##
=======================================
  Coverage   95.61%   95.61%           
=======================================
  Files          29       29           
  Lines        1004     1004           
=======================================
  Hits          960      960           
  Misses         44       44           

Impacted Files	Coverage Δ
src/utility/ota/OTALogic.cpp	98.03% <ø> (ø)

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[github-actions]

Memory usage change @ 63e85b2

Board	flash	RAM for global variables
arduino:samd:mkrgsm1400	❔ -40 - +1176	🔺 +16 - +16
arduino:samd:mkrwan1300	0 - 0	0 - 0
arduino:samd:mkrwifi1010	❔ -48 - +1200	🔺 +16 - +16
arduino:samd:nano_33_iot	❔ -48 - +1200	🔺 +16 - +16

Click for full report table

Board	examples/ArduinoIoTCloud-Advanced flash	examples/ArduinoIoTCloud-Advanced RAM for global variables	examples/ArduinoIoTCloud-Basic flash	examples/ArduinoIoTCloud-Basic RAM for global variables	examples/utility/ArduinoIoTCloud_Travis_CI flash	examples/utility/ArduinoIoTCloud_Travis_CI RAM for global variables	examples/utility/Provisioning flash	examples/utility/Provisioning RAM for global variables
arduino:samd:mkrgsm1400	1176	16	1176	16	464	16	-40	16
arduino:samd:mkrwan1300	0	0	0	0	0	0
arduino:samd:mkrwifi1010	1200	16	1200	16	496	16	-48	16
arduino:samd:nano_33_iot	1200	16	1200	16	488	16	-48	16

Click for full report CSV

Board,examples/ArduinoIoTCloud-Advanced<br>flash,examples/ArduinoIoTCloud-Advanced<br>RAM for global variables,examples/ArduinoIoTCloud-Basic<br>flash,examples/ArduinoIoTCloud-Basic<br>RAM for global variables,examples/utility/ArduinoIoTCloud_Travis_CI<br>flash,examples/utility/ArduinoIoTCloud_Travis_CI<br>RAM for global variables,examples/utility/Provisioning<br>flash,examples/utility/Provisioning<br>RAM for global variables
arduino:samd:mkrgsm1400,1176,16,1176,16,464,16,-40,16
arduino:samd:mkrwan1300,0,0,0,0,0,0,,
arduino:samd:mkrwifi1010,1200,16,1200,16,496,16,-48,16
arduino:samd:nano_33_iot,1200,16,1200,16,488,16,-48,16