Squashed 'site_scons/' changes from 2b57224..6e314f8

6e314f8 Remove unused toolchains parameter
43288f2 Update README to add GCOVR to the dependencies

git-subtree-dir: site_scons
git-subtree-split: 6e314f84961babb29f43681ecf952cf4b1bb739b
Merge commit '06ed68b7a6ed137f01600322b84f17b6ef0982e0'