Provide complete support for app_config

[prp]

We need to improve the support for app_config JSON files, so we can use it to specify the full configuration. Currently large parts still need to be set using environment variables for sgx-lkl-run because the app_config doesn't have equivalent settings.

Blocker for https://github.com/lsds/sgx-lkl/issues/144.

Depends on https://github.com/lsds/sgx-lkl/issues/97.

Related to https://github.com/lsds/sgx-lkl/issues/207 and https://github.com/lsds/sgx-lkl/issues/267.

[davidchisnall]

We also have a couple of things that are not part of our confit yet, but should be. In particular, the valid CPUID leafs and the contents of /proc/cpuinfo.