Allow to skip SGX token generation

[dimakuv]

Description of the changes

SGX Launch Token (aka EINITTOKEN) is generated right-before Gramine launch via the gramine-sgx-get-token tool. For DCAP platforms, the SGX token is unused, so there is no need to generate it. This commit introduces a GSC switch --skip-token-generation for this.

This is useful for DCAP platforms (one less file in the Docker container), and enables such scenarios as docker run --read-only.
Also, skipping running gramine-sgx-get-token paves a path to removing Python as a run-time dependency for GSC-built Docker images.

Related PR in core Gramine: gramineproject/gramine#363

How to test this PR?

A Python example from docs with ./gsc build --skip-token-generation --insecure-args python test/generic.manifest. You'll see that gramine-sgx-get-token tool is not called during the last step (docker run).

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This change is 

[mythi]

I tested this fix and it works well!

FWIW, the resulting apploader.sh looks like this but that's probably fine :-)

then

    gramine-sgx /entrypoint  "${@}"
else

[dimakuv]

FWIW, the resulting apploader.sh looks like this but that's probably fine :-)

I think we can live with it :) Btw, we'll need to remove set -x from the apploader script; it's annoying to see all those logs "hey user, look what I'm executing now".

[dimakuv]

Closing this PR as it is considered as a stop-gap solution, and also it is not high priority. We better fix it properly.

See gramineproject/gramine#363 for details.