nrf: Added support for writing with nrfjprog on all nrf boards

`tinygo flash -programmer command` to write using nrfjprog

targets/pca10056.json

@@ -2,7 +2,6 @@
 	"inherits": ["nrf52840"],
 	"build-tags": ["pca10056"],
 	"flash-method": "command",
-	"flash-command": "nrfjprog -f nrf52 --sectorerase --program {hex} --reset",
 	"msd-volume-name": "JLINK",
 	"msd-firmware-name": "firmware.hex",
 	"openocd-interface": "jlink",