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been replaced by more specific properties.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/hifive1b.json

@@ -4,7 +4,7 @@
 	"ldflags": [
 		"-T", "targets/hifive1b.ld"
 	],
-	"flash": "{hex}",
 	"flash-method": "msd",
-    "flash-msd-volume-name": "HiFive"
+	"msd-volume-name": "HiFive",
+	"msd-firmware-name": "firmware.hex"
 }