Currently the build supports skips for leaf nodes: archive creation and OS
packages.  This adds skip options for folder cleanup and source build
folders.  This is intended to support two scenarios:

1) Debugging.  Creating the build source folders and compiling assets
takes up the bulk of build time.  This can make testing package
creation changes time consuming.

2) Build resuming.  When creating docker images, we want the option to
drop an archive in the target folder and run only the docker build step.

Three flags have been added:
```
--skip-initialize Don't clean the target and build folders
--skip-generic-folders Don't create the base source folders
--skip-platform-folders Don't create the platform specific folders
```

The build syntax for this is verbose and probably due for refactoring.
The long term goal is to migrate this to bazel, and the changes here are
intended as stopgap until then.

Testing - standalone Docker image
```
yarn kbn clean
yarn kbn bootstrap
mv ~/Downloads/kibana-8.0.0-SNAPSHOT-linux-x86_64.tar.gz target
node scripts/build \
  --no-oss \
  --skip-initialize \
  --skip-generic-folders \
  --skip-platform-folders \
  --skip-archives \
  --docker-images \
  --skip-docker-ubi \
  --skip-docker-contexts
```