setup-alpine.in: only enable acpid if machine has ACPI support

The acpid init.d daemon should not be enabled on machines that do not actually have ACPI support - this includes most Arm SBCs and occasionally some x86/x86_64 machines (including Firecracker VMs).

If acpid is enabled in such situations then it will fail to start and the OpenRC service will show as crashed.

Only enable the acpid service if the /sys/firmware/acpi directory exists which indicates the machine does have ACPI support.

Closes aports#12290 (closed) and aports#13700 (closed)