This change adds support for `tinygo gdb` on the PCA10056.
The board is normally flashed with the MSD programmer. Debugging needs a
real debugger interface however, which is relatively simple by just
adding the right OpenOCD configuration.
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been replaced by more specific properties.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.
