Hi Gary Tivey
SDK 2.x doesn't need to compile platform library, If you have to compile libsdk_platform.a you probably have SDK 1.x version.
If you want to update to the latest version of SDK you can download it here; Welcome to MCUXpresso | MCUXpresso Config Tools , if you are using KDS, Also, you will need Service Pack: Eclipse add-on to add Kinetis SDK V2.x Project Wizard v2.0.1, You can find it in the KDS download page.
If you want to use the version that you have, you need to first compile libsdk_platform.a library, it is located in the path: <KSDK_path>\lib\ksdk_platform_lib\<your_IDE>\<your_device>
Hope this information could help you
Have a great day,
Jorge Alcala
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