Revert "Merge pull request #132 from bgamari/update-libopencm3"

This reverts commit cbe69b3d0b0828e25850e9f3432848d99ec0865d, reversing
changes made to 2f2ca2e630651d97d385d9044149f2f13dd28d05.

I shouldn't have merged that ancient pull request that somehow was
reopened.