make `baseComposite` to understand relative paths

[romanlv]

baseComposite is a required part of running a miniapp in our case and ern run-ios is included in package.json scripts

Right now, we are forced to use external tool realpath which has to be installed separately e.g.

"scripts": {
  "ios": "COMP=`realpath $(pwd)/../ern-composite` && ern --baseComposite=$COMP run-ios -u",
}

it would be great to provide relative path, so realpath hack is not required

[romanlv]

also, for some unknown reasons yarn ios command fails, but npm run ios works It's probably due to the fact that ERN uses yarn internally