travis: run build-tools.sh with CMAKE_BUILD_TYPE=Release

[marc-hb]

For some reason gcc prints more warnings this way and of course CI is
meant to catch warnings as soon as possible.

[marc-hb]

The warnings in https://travis-ci.org/github/thesofproject/sof/jobs/727258703 are fixed by PR #3412

[lgirdwood]

@marc-hb I'm assuming CI is using latest GCC 9.x as this will highlight more code issues than older GCC or XCC.

[marc-hb]

The warnings in https://travis-ci.org/github/thesofproject/sof/jobs/727258703 are fixed by PR #3412

-Werrs are now fixed after rebasing on top of now merged #3412 :
https://travis-ci.org/github/thesofproject/sof/builds/727431437

@marc-hb I'm assuming CI is using latest GCC 9.x as this will highlight more code issues than older GCC or XCC.

I don't know what the docker image uses right now, I use a plain Ubuntu 20.04 and it has gcc 9.3. This is how I found the compilation issues fixed in #3412 I generally don't follow where the docker image is at.

thesofproject/linux/.travis.yml doesn't use docker either (it does no cross-compilation either). It was recently upgraded to plain Ubuntu 20.04 and gcc 9.3 too (+ some clang version).

I understand the convenience of docker for not having to build toolchains (which is not long but a bit tedious and error-prone) for new users and making sure they all use the same ones. However for me it's more an additional layer of indirection/obfuscation when you want to test and fix build issues.

Unless the docker image uses something special for native compilation too, using it to build tools/ seems at best pointless. Happy to remove this indirection in a later PR if you'd like me to.

[marc-hb]

You can see that Travis default distribution is Xenial 16.04: https://travis-ci.org/github/thesofproject/sof/builds/727431437/

In the same logs you can see 16.04 has a very old gcc 5.4 by default. Inside the docker image CMake conveniently reports that gcc is currently 7.5

Switching to 20.04 (or anything else) in thesofproject/linux/.travis.yml was a one-line change.

[lgirdwood]

@xiulipan can you confirm what GCC version docker/travis is using for CI

[xiulipan]

I think we do not need the change to the docker scripts.