13 sloc reduction

Signed-off-by: Riku Viitanen <riku.viitanen@protonmail.com>

Signed-off-by: Riku Viitanen <riku.viitanen@protonmail.com>

moved cmake files into a separate build directory.
this can just be deleted for the source release.

might as well use cmake for the actual build too.
that makes repeated builds faster for some reason.

Signed-off-by: Riku Viitanen <riku.viitanen@protonmail.com>

Tested on a Nucleo-F042K6.

That has an onboard stlink:
`st-flash --format ihex write bin/serprog_stm32/serprog_nucleo-f042k6.hex`

The usb port used for flashing is separate, its is exposed on
the pin header instead. Check boards/nucleo-f042k6.h for usb pinout.

Signed-off-by: Riku Viitanen <riku.viitanen@protonmail.com>