Exposes uf2.FamilyId from build.zig (#16)

Co-authored-by: Felix "xq" Queißner <git@random-projects.net>

tools/uf2/build.zig

@@ -5,6 +5,8 @@ const FileSource = std.Build.FileSource;
 
 const uf2 = @import("src/uf2.zig");
 
+pub const FamilyId = uf2.FamilyId;
+
 pub fn from_elf(dep: *Dependency, exe: *CompileStep, opts: uf2.Options) FileSource {
     std.debug.assert(!opts.bundle_source); // TODO: bundle source in UF2 File
     const b = dep.builder;