Ignore optiboot build artifacts

Object files, elf/hex/lst outputs, and the generated baudcheck
script were showing up as untracked clutter.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitignore

@@ -1,2 +1,9 @@
 /*.md
 !/README.md
+
+# optiboot build artifacts
+optiboot/*.o
+optiboot/*.elf
+optiboot/*.hex
+optiboot/*.lst
+optiboot/baudcheck.tmp.sh