Add simple hook support as a start.

Closed Carlo Landmeter requested to merge github/fork/Magicloud/hooks into master

Created by: Magicloud

Hooking helps customizing stage 1 behavior without hack vanilla image. Decoupling things from source level means less trouble handling upstream (Alpinelinux) upgrading.

Three hooks are added.

  1. After parsing kernel parameters. So hooks can have them all, and unset some to control following process.
  2. Before switch_root when root is specified.
  3. Before switch_root when root is not specified.

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