Minimal porting instructions?

[SonoSooS]

I've been looking at the ports directory for a while now, and I have either missed it, or I can't find an obvious way to begin to port a new device.

I have an ARM7TDMI device with no GPIO^[1], no conventional communication ports^[2], and no USB either.
I do however have 32Megabytes of rewritable ROM, and 256k+32k of RAM, and a way to communicate with the device via a bugged UART hardware, and also some buttons (read-only).

I also need to use a custom linker script to make it bootable on real hardware.

Where should I start?

^[1] It has GPIO, but that disables UART, and it's also very limited GPIO with no features (three output, one input, input only has low-to-high interrupt).
^[2] I have made an UART adapter to make use of the bugged UART hardware, but it's no use besides some extremely basic REPL.