[BPI-R4] package adguardhome - alternative update script

Found in OpenWRT forum and might be useful as long as package is disappeared from official build image / snapshot service.

I’ve modified this script to fit the needs of my BPI-R4 but propably it should fit any generic OpenWRT device with overlay filesystem:

(there are some references left to GL.inet specific forums. Should not have impact on function.)

(Original script for GL.inet devices (OS v4+): glinet.forum/scripts/update-adguardhome at main · Admonstrator/glinet.forum · GitHub)

(build image / snapshot service - github issue here: adguardhome: package missing in snapshots/packages/aarch64_cortex-a53 tree · Issue #23933 · openwrt/packages · GitHub)