3748: expose TR181-compliant methods of managing firmware banks

For context, see https://project.iopsys.eu/issues/3748

The overarching goal of this is to expose an asynchronous ubus function upgrade() for the fwbank object. The purpose of this is to give the ability to manage the firmware banks of the system (fwbank.dump()), and install a firmware image on them (fwbank.upgrade()) as needed.

This MR:

• Improves the robustness the dump method of the fwbank object as well as is possible right now:
  • Make it initially work again (5a772aae), depend on the system layout helpers (974b6c89), avoid meddling with jshn's internal state (76e968db)
  • Implement the status field for a given firmware bank (694a5ade).
  • Move the implementations of the fwver field into its own functions (19872a67). The big problem is deciding how and where to store the fwver (firmware version) fields of both firmware banks; right now the pragmatic decision has been made to store them on the toplevel of both overlay partitions (<overlay>/fwbank_${bank_id}/fwver) (4a571b06). This will be iterated upon, starting by storing the fwver field in the FIT image, to avoid mounting the other bank's root file system.
• implements the upgrading of a firmware bank with a given image, through the ubus method fwbank.upgrade(). This function is implemented asynchronously with the help of the nohup busybox built-in. (9599fe96)
• simplifies the set_bootbank method of the ubus object (57f0c85b), as the existing implementation seemed a bit convoluted.

Dependencies

This MR depends on

• the nohup utility being compiled with busybox: iopsys!86 (merged)
• the sysupgrade lock path being fixed in one place, for the asynchronous fwbank.upgrade() method to work well (as the sysupgrade lock is tested): !31 (merged)

Testing and example output

root@iopsys:~# ubus call fwbank upgrade '{"uri":"/tmp/last.pkgtb","auto_activate":"true", "bank_id": 1}'
root@iopsys:~# ubus call fwbank set_bootbank '{"bank":"0"}'
root@iopsys:~# ubus call fwbank dump
{
	"bank": [
		{
			"name": "A",
			"id": 0,
			"active": true,
			"boot": true,
			"upgrade": false,
			"fwver": "DG400PRIME-A-IOPSYS-6.1.0BETA2-210329_1302",
			"swver": "6.1.0BETA2",
			"status": "Active"
		},
		{
			"name": "B",
			"id": 1,
			"active": false,
			"boot": false,
			"upgrade": true,
			"fwver": "DG400PRIME-A-IOPSYS-6.1.0BETA2-210326_1753",
			"swver": "6.1.0BETA2",
			"status": "Available"
		}
	]
}