Add support to boot images via iPXE

[schrodit]

How to categorize this PR?

/area os /kind enhancement /platform equinix-metal

What this PR does / why we need it:

Adds the ability to boot custom images via iPXE (https://deploy.equinix.com/developers/docs/metal/operating-systems/custom-ipxe/). This is useful if

• a different os than the officially maintined images should be used
• a specific version of a OS should be pinned. This is needed because for example for Flatcar, Equinix is currently
  • rarely updating their channels (stable, beta, ...)
  • you are stick to the version Equinix chooses
  • the used version is intransparent
  • The version differs from DC to DC

Special notes for your reviewer:

Requires https://github.com/gardener/machine-controller-manager-provider-equinix-metal/pull/18 to be merged and released.

Release note:

Add support to specify custom images via iPXE boot.

[gardener-robot]

@schrodit Labels area/todo, kind/todo do not exist.

[gardener-robot]

@schrodit Thank you for your contribution.

[gardener-robot-ci-3]

Thank you @schrodit for your contribution. Before I can start building your PR, a member of the organization must set the required label(s) {'reviewed/ok-to-test'}. Once started, you can check the build status in the PR checks section below.

[gardener-robot]

@schrodit You need rebase this pull request with latest master branch. Please check.