Save resources in the build and deployment process

[probonopd]

Objectives:

• Cache SDKs for all architectures to produce less traffic and build faster
• Upload only the latest version to gh-pages (do not save history) to decrease repo size

For the caching, it looks like we need to wget the SDKs for all platforms into cache/ regardless of the platform we are currently building.

For gh-pages, we should use something along the lines of https://github.com/mazerte/test-footguard/blob/master/scripts/gh-pages.sh like

cd dist
rm -rf .git
git init
# git remote add origin $(git config --get remote.origin.url) # > /dev/null 2>&1
git remote add origin https://${TOKEN}@github.com/${USER}/${REPO}.git # > /dev/null 2>&1
git push origin --delete gh-pages > /dev/null 2>&1
git checkout -b gh-pages
git add --all
git commit -m "$message"
git push origin gh-pages -f > /dev/null # 2>&1
cd - 

However, this can only work if we build all platforms in one go (rather than parallel), because we basically re-generate gh-pages from scratch every time.

Pull requests are welcome.