Update `libopencm3`?

Why should we?
----------------

- SPI improvements?
- Allowing to `make` for just one architecture? (stm32/f1)
- Anything else that could be interesting?

Things to consider
-------------------

- Our current `stm32f103x8.ld` file [may be different in the latest version of libopencm3](https://github.com/libopencm3/libopencm3/blob/master/lib/stm32/f1/stm32f103x8.ld)?