Resin-Supervisor: test config.txt changes

[pcarranzav]

To be tested only on the Raspberry Pi family.
Test steps:

• Set a RESIN_HOST_CONFIG_foo variable
• Expected result: Logs show the variable is being applied. The device reboots and has a foo variable in config.txt correctly set.
• Change the variable to a different value.
• Expected result: same as before, now after the reboot config.txt will show the updated value.
• Delete the variable
• Expected result: after reboot, the device no longer shows the foo variable in config.txt

[horia-delicoti]

Useful documentation: https://docs.resin.io/configuration/advanced/