Currently you can't install yarn using yum so there doesn't seem to be an easy way to create a config that installs it before asset pre-compilation.
You can customize packages that are installed and commands that are run on deploy with
For yarn, I created a file with the following commands which install a recent node version and yarn:
# .ebextensions/yarn.config commands: 01_install_node: command: | sudo curl --silent --location https://rpm.nodesource.com/setup_8.x | sudo bash - sudo yum -y install nodejs 02_install_yarn: # don't run the command if yarn is already installed (file /usr/bin/yarn exists) test: '[ ! -f /usr/bin/yarn ] && echo "Yarn not found, installing..."' command: | sudo wget https://dl.yarnpkg.com/rpm/yarn.repo -O /etc/yum.repos.d/yarn.repo sudo yum -y install yarn