netbird: switch to profile-based configuration

netbird: switch to profile-based configuration

Add support for `netbird` profiles feature [1] (introduced in 0.52.2). Use
`NB_STATE_DIR="/root/.config/netbird"` in the init file instead of the
previous `NB_CONFIG="/etc/netbird/config.json"`, and update Makefile
configuration paths accordingly.

Rationale: `netbird` saves state/configuration under `/var` by default. On
OpenWrt, `/var` is a symlink to `/tmp` (a volatile location), which would
cause config loss after reboot or reflash. Using `/root/.config/netbird`
avoids this, it's a valid upstream directory not used by default.
`netbird` will migrate existing configurations.

This change will not be backported to preserve the current meaning of
"stable" in OpenWrt and to avoid (unlikely but possible) breakage of
existing installations.

[1]: https://docs.netbird.io/how-to/profiles

Signed-off-by: Wesley Gimenes <[email protected]>