Support different tools for OEM unlock in the installer

Summary

Currently the easy installer has a script oem-unlock that uses "heimdall" as tool to do the unlocking. As far as I know this only works for Samsung devices.

The problem

Hardcoding "heimdall" for OEM unlock makes it difficult to add new devices to the installer.

Solutions

Possible fixes

I could imagine two solutions:

1. Use one oem-unlock script and add a parameter to switch between tools (heimdall, fastboot, ...) inside the script.
2. Create multiple scripts like oem-unlock-fastboot and rename the existing script to oem-unlock-heimdall and use the correct one in the device specific YAML files.

I'm clearly in favor of option two.