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been replaced by more specific properties.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/feather-m0.json

@@ -1,8 +1,8 @@
 {
     "inherits": ["atsamd21g18a"],
     "build-tags": ["sam", "atsamd21g18a", "feather_m0"],
-    "flash": "{uf2}",
     "flash-1200-bps-reset": "true",
     "flash-method": "msd",
-    "flash-msd-volume-name": "FEATHERBOOT"
+    "msd-volume-name": "FEATHERBOOT",
+    "msd-firmware-name": "firmware.uf2"
 }