Only consider bpf.o files in $prefix/lib/firmware/hid/bpf
Instead of hardcoding /usr and /usr/local let's work with the meson-provided bpfdir (from the prefix) and use that as our only default lookup path.
This means installing in different prefixes will work independently but that's kinda the point. If there's a use-case for multiple lookup directories we can sort this out then.