[Gen2] Fix sprintf compile err on gcc9

[keeramis]

Problem

Compile error when building Electron after updating to gcc compiler 9.2.1

error: 'sprintf' may write a terminating nul past the end of the destination

Solution

Use snprintf instead of sprintf to give number of bytes.

Steps to Test

Compiling Electron on gcc 9.2.1 likely triggers this error.

Example App

void setup() {
}

void loop() {
}

References

ch55930

---

Completeness

• User is totes amazing for contributing!
• Contributor has signed CLA (Info here)
• Problem and Solution clearly stated
• Run unit/integration/application tests on device
• Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)

[keeramis]

Worked with Julien and made sure we are also fixing the buffer sizes allocated for the bands.

[technobly]

As discussed, on-device tests should be updated and run: https://github.com/particle-iot/device-os/tree/develop/user/tests/app/band_select

[keeramis]

@technobly Fixed on-device tests 👍