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been replaced by more specific properties.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.
This commit avoids setting the working directory to the TinyGo root when
invocating Clang. This helps to weed out issues before we add support
for bundling Clang in a release.
This avoids a ton of duplication and makes it easier to change a generic
target (for example, the "cortex-m" target) for all boards that use it.
Also, by making it possible to inherit properties from a parent target
specification, it is easier to support out-of-tree boards that don't
have to be updated so often. A target specification for a
special-purpose board can simply inherit the specification of a
supported chip and override the properites it needs to override (like
the programming interface).
