athp: cleanup modules build Makefiles

[bzfbd]

The Makefiles had a lot of copy and paste and especially CLEANFILES was wrong in some of them. Rather than keeping the duplicate logic everywhere, put it into a common Makefile.inc and only have the per-firmware kmod specifics in the individual Makefiles. This will also allow us to change the way we are building the firmware(9) kernel modules in a single place.

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