Hi David,
Freemaster is debug monitor and data visualization tool. It is for displaying variables changing on an oscillooscope-like display. We don't use it as bootloader.
If you want to use bootloader program flash via SCI, you can refer
https://www.nxp.com/docs/en/application-note/AN2548.pdf
or
https://www.nxp.com/docs/en/application-note/AN3275.pdf
or
https://www.nxp.com/docs/en/application-note/AN4258.pdf
You can also search out related bootloader project from nxp web site
Have a great day,
Jennie Zhang
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