targets: refactor flash/gdb target 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been replaced by more specific properties.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.

targets/arduino-nano33.json

@@ -1,6 +1,6 @@
 {
     "inherits": ["atsamd21g18a"],
     "build-tags": ["sam", "atsamd21g18a", "arduino_nano33"],
-    "flash": "bossac -d -i -e -w -v -R --port={port} --offset=0x2000 {bin}",
+    "flash-command": "bossac -d -i -e -w -v -R --port={port} --offset=0x2000 {bin}",
     "flash-1200-bps-reset": "true"
 }