Improve naming of Serial-TL PhyParams

[jerryz123]

This improves the naming of the Serial-TL parameterization objects to make it clearer what is generated.

The Decoupled{Internal/External}SyncPhy implementation was determined to be deadlocking, although the condition under which deadlock could occur is effectively impossible on real silicon ... deadlock is only possible if both sides fully saturate the channel and run at the same frequency.

This PR adds a loud warning in the generator when building with that form of flow control.... it should still be safe for test chips, as the chip-FPGA link is always throttled by the FPGA, or by the UART-TSI onto the FPGA.

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