tracker: allow use in 48K model with external AY addons

[jorgegv]

There are apparently AY interfaces for 16K/48K models which are compatible with the one used in 128K models, so tracker music could be added to 48K builds. Currently, TRACKER feature depends on ZX_TARGET_128 feature, but it is a hardcoded dependency which may not be necessary.

Example AY interface: https://www.bytedelight.com/?product=zx-ay-external-stereo-audio-interface-for-all-zx-spectrum-models

Input from the manufacturer:

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Hey Jorge,

It uses (responds to) exactly the same ports as internal AY chips.
The reason I developed it is to have an easy way to achieve crisp stereo sound, and to add AY sound to 48K ZX Spectrums.

Unfortunately the AY chip itself is getting rare and expensive, so ‘an easy way’ is getting less easy (or affordable).

AY clone chips are coming though, which will be a good substitute, and less expensive.

Best regards,

Ben Versteeg
ByteDelight

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Previous discussions on this issue were in #103 .

Tasks:

• Move tracker code from banked/128 to banked/common so that it can be compiled in 48K or 128K mode
• Move generated and tracker player code to build/generated/banked/common
• Remove hard dependency of TRACKER on ZX_TARGET_128
• Test in 48K mode with AY hardware available (may be a 128 running in 48K mode?)
• Document everything