sencha: refactor cheriot-testbench and add cheriot-examples - refactor cheriot-testbench support into cheriot-tests.mk - add cheriot-examples.mk that builds the cheriot-rtos examples; there are no targets (yet) to run the firmware images but one can use the sim_cheriot_renode shell function Bypass-Presubmit-Reason: no sencha CI tests Change-Id: Idea8d5b2b4b2be1a0926ebb4447b06eb0d6259ad