Integrate buildroot and add support for QEMU Linux target

[metin-arm]

Commits:

• 918c19c - utils/ssh: Try to free up resources during client creation
• 3b7fb5b - target: Introduce make_temp() for creating temp file/folder on target
• 1253903 - tests/test_target: Test more targets
• 82a722d - tools/buildroot: Add support for generating Linux target system images
• a4b3547 - target: Implement target runner classes

[setrofim]

This should probably be implemented as a Platform rather than a Target, as this handles "hardware" rather than OS-level functionality; And that way, the same code can be used for Android targets running on QEMU as well.

[douglas-raillard-arm]

@setrofim I'm not sure platform would be appropriate either, we want an object that starts its life before the Target and survives after. Platform seems to be more for managing platform-specific interaction with a device (such as rebooting) more than spawning the device itself.

[setrofim]

we want an object that starts its life before the Target and survives after

A Platform is passed a a parameter on Target creation, so it is already assumed to exist before the Target (only if one is not provided, the Target will then create a "default" one). And the Target doesn't do anything to "terminate" a Platform, so there is no reason why the latter can't persist after the former stopped existing.

Platform seems to be more for managing platform-specific interaction with a device (such as rebooting) more than spawning the device itself.

Only because a device is not normally "spawned". However the interactions can include the initial target setup prior to execution (e.g. flashing images -- see VExpress platform). Support for Gem5 simulation (conceptually similar to QEMU) is implemented as a Platform and that includes optionally starting the simulation.

[metin-arm]

@douglas-raillard-arm, https://github.com/ARM-software/devlib/pull/677 is filed to address your comments except open questions here.