modules: update openwrt

a7fb589e8a image: always rebuild kernel loaders
5db6914f7c mpc85xx: p1010: make TP-Link WDR4900 v1 build again
f7a43e4606 mpc85xx: add SPI kernel loader for TP-Link TL-WDR4900 v1
c1fcca50ba ramips: fix ZyXEL NWA55AXE model name
2050bc4f64 ramips: add support for ZyXEL NWA50AX / NWA55AXE
bc8e24c654 busybox: nslookup: ensure unique transaction IDs for the DNS queries