Bump to SGX SDK 2.12 and mention also Ubuntu 20.04

[g2flyer]

What this PR does / why we need it:

Make sure for release that we support latest SGX version; also enables us to provide support for Ubuntu 20.04 (see notes in README.md for some small config.override.mk change needed.)

Notes:

• It still should work on a host having older SGX SDK, e.g., 2.11, just not on Ubuntu 20.04. So not hurry to upgrade your various test machines.
• there is also a newer version of sgxssl ( lin2.11_1.1.1.g vs currently used lin2.10_1.1.1.g). As this version changed (pretty much undocumented short of code) pthread stuff and breaks pdo crypto, i didn't bump that version.

Which issue(s) this PR fixes:

Next iteration of #411

Special notes for your reviewer:

Does this PR introduce a user-facing changes and/or breaks backward compatability?:

[bvavala]

lg.
Importantly, travis/docker tests with the 2.12 SDK, but locally the 2.10 SDK still works.