add continuous integration

[jovanbulck]

eg Travis-CI to at least make sure everything compiles on an up-to-date system. This means we should also patch, build, and install the SGX-SDK.

We can also consider running at least some tests from the app directory in the CI environment.

[jovanbulck]

Elementary CI is now integrated in GitHub Actions to:

1. Build and load /dev/sgx-step
2. Patch and build the linux-sgx Intel SDK and PSW
3. Build libsgxstep and sample applications

Ideally, CI should also execute at least some of the sample applications to test some of the features of the framework that do not rely on SGX -- should look into how feasible it is test APIC manipulations in a CI VM environment