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.
This makes debugging on the HiFive1 rev B much easier:
tinygo gdb -target=hifive1b examples/echo
Using JLinkGDBServer as I couldn't figure out how to do it with OpenOCD.
Setting the linker script as one property (instead of as part of the
generic ldflags property) allows it to be overriden.
This is important for the SoftDevice on Nordic chips, because the
SoftDevice takes up a fixed part of the flash/RAM and the application
must be flashed at a different position. With this linkerscript option,
it is possible to create (for example) a pca10040-s132v6 that overrides
the default linker script.
Instead of specifying explicit commands, most of these commands have
been replaced by more specific properties.
This is work that will be necessary for an eventual -programmer flag to
the compiler, with which it is possible to select which programmer to
use to flash or debug a chip. That's not very useful for boards that
already include a programmer or bootloader for that purpose, but is very
useful for novel boards or single-purpose boards that are not already
included in TinyGo.
