I'm not entirely convinced Swift SDKs are the right answer for embedded platforms. The primary reason to have a Swift SDK is a need for a special libc and dependencies like swift-corelibs-dispatch and swift-corelibs-xctest that are hard to build as a package. Another potential use case is the ability to define test runners and debuggers, but this latter possibility hasn't been explored yet.
Since you don't immediately need either of those in this case, if you can fit all the required platform things into a separate package it should be much easier for users to depend on it. That also makes it highly customizable and easy to update in one go, as opposed to adding an out-of-package Swift SDK dependency.
If you need to add unsafe compiler/linker flags to such "embedded platform support" package, please let us know. I hope we'll be able to work around or find fixes for those new requirements.