all: add a flag to the command line to select the serial implementation

This can be very useful for some purposes:

  * It makes it possible to disable the UART in cases where it is not
    needed or needs to be disabled to conserve power.
  * It makes it possible to disable the serial output to reduce code
    size, which may be important for some chips. Sometimes, a few kB can
    be saved this way.
  * It makes it possible to override the default, for example you might
    want to use an actual UART to debug the USB-CDC implementation.

It also lowers the dependency on having machine.Serial defined, which is
often not defined when targeting a chip. Eventually, we might want to
make it possible to write `-target=nrf52` or `-target=atmega328p` for
example to target the chip itself with no board specific assumptions.

The defaults don't change. I checked this by running `make smoketest`
before and after and comparing the results.

1 files changed, 1 insertions, 0 deletions

diff --git a/targets/pca10031.json b/targets/pca10031.json
index cccae828a..8af365cc8 100644
--- a/targets/pca10031.json
+++ b/targets/pca10031.json
@@ -1,6 +1,7 @@
 {
 	"inherits": ["nrf51"],
 	"build-tags": ["pca10031"],
+	"serial": "uart",
 	"flash-command": "nrfjprog -f nrf51 --sectorerase --program {hex} --reset",
 	"openocd-interface": "cmsis-dap"
 }