owm: add "hardwareinfo" node for modelname and system-platform

[SvenRoederer]

• cleans the mixed use of hardware data and runtime data in the "system" node as mentioned in https://github.com/freifunk-berlin/firmware/issues/382
  • uses the "system" node only for runtime data
  • adds a new "hardwareinfo" node with a "model" and "system" field model: usually the marketing name from the type-label system: general hardware platform / SoC / reference-design
• drops some legacy compatibility code added in https://github.com/freifunk-berlin/firmware-packages/commit/222efe80f1c578a3a8f09fa7244fcab9f5c52395 to support a statistics-page that have already shut down
• as the OWM-backend has been renewed recently, dropping the legacy code now seems the best moment

new JSON node

hardwareinfo |
  openwrt_boardname | "tplink,tl-wr841n-v13"
  model | "TP-Link TL-WR841N v13"
  system | "MediaTek MT7628AN ver:1 eco:2"
system |
  uptime |
    0 | 4692
  loadavg |
    0 | 0.12353515625

old JSON node

system |
  sysinfo |
    0 | "system is deprecated"
    1 | "TP-Link EAP225-Outdoor v1"
  uptime |
    0 | 651450
  loadavg |
    0 | 0.0244140625