teach x1plusd.ota to download OTA images

[jwise]

In combination with #260, x1plusd can now download and install an OTA of its own. After installing this version (including the kexec_ui changes from #260), try something like:

[root@BL-P001-sdcard:~]# x1plus-test-settings.py ota.enabled=True
[root@BL-P001-sdcard:~]# x1plus-test-settings.py ota.json_url=https://moroso.emarhavil.com/~joshua/x1ota/ota.json
[root@BL-P001-sdcard:~]# dbus-send --system --print-reply --dest=x1plus.x1plusd /x1plus/ota x1plus.ota.GetStatus string:'{}'
[root@BL-P001-sdcard:~]# dbus-send --system --print-reply --dest=x1plus.x1plusd /x1plus/ota x1plus.ota.Download string:'{"base_firmware": true}'
[root@BL-P001-sdcard:~]# dbus-send --system --print-reply --dest=x1plus.x1plusd /x1plus/ota x1plus.ota.GetStatus string:'{}'
(... repeat until status is IDLE again and both ota_is_downloaded and ota_base_is_downloaded ...)
[root@BL-P001-sdcard:~]# dbus-send --system --print-reply --dest=x1plus.x1plusd /x1plus/ota x1plus.ota.Update string:'{}'

[bani6809]

it is worth noting that x1plus ota is fundamentally incompatible with lan+shield mode. the UI should probably make this obvious.

[bani6809]

lgtm. tested, seems to work