build-yocto: Define envvars to avoid UTF8 error

[gmacario]

Starting with Yocto morty, you will get the following error when doing bitbake xxx using Docker image gmacario/build-yocto:

Please use a locale setting which supports utf-8

The issue was only partially resolved with PR #250

Additionally, the following environment variables must be set inside the build script

export LC_ALL=en_US.UTF-8
export LANG=en_US.UTF-8
export LANGUAGE=en_US.UTF-8

See gmacario/my-agl-pipelines#9 and #243

We should define those envvars directly inside the Docker image to avoid cluttering the build scripts.