The leading - makes it rather inconsistent with the majority of other
projects around the world. Use the form everyone else uses.
To solve this, properly pass prefix to inner makes as was always
intended.
Fixes: https://github.com/libopencm3/libopencm3/issues/1058
Thoughts: should this be a "sam0" family rather than samd? (Much like Atmel's
own software package lumps all the cortex-m0+ devices in one family)
This was enough to get a basic blinky working at least.