[Nexthop] Add usb0 and eth0.4088 NetworkManager profiles

[siddarth-nexthop]

Pre-submission checklist

• I've ran the linters locally and fixed lint errors related to the files I modified in this PR. You can install the linters by running pip install -r requirements-dev.txt && pre-commit install
• pre-commit run

Summary

Add NetworkManager profiles for the two private host-to-BMC links (usb0 and eth0.4088) so they are brought up automatically at boot instead of being configured at test time.

• usb0 profile: static fe80::2/64, matching the BMC's fe80::1 peer.
• eth0.4088 profile: VLAN 4088 on eth0, link-local IPv6.
• config.sh: widen the existing chmod 600 to cover all *.nmconnection profiles so the new files are loaded by NetworkManager.

Test Plan

Verified end-to-end on a Minipack3 DUT, freshly PXE-prepared with the build produced by this change. No manual nmcli commands were run after boot.

On the resulting image:

• NetworkManager profiles present under /etc/NetworkManager/system-connections/ with mode 0600
• NetworkManager loaded eth0, usb0, and eth0.4088 automatically
• usb0 came up with static fe80::2/64
• eth0.4088 was created on eth0 with IPv6 link-local
• SSH to the BMC worked over both usb0 and eth0.4088

Representative output:

# ls -la /etc/NetworkManager/system-connections/
-rw-------. 1 root root 156 May 21 22:29 eth0.4088.nmconnection
-rw-------. 1 root root 112 May 21 22:29 eth0.nmconnection
-rw-------. 1 root root 139 May 21 22:29 usb0.nmconnection

# nmcli connection show
NAME       UUID                                  TYPE      DEVICE
eth0       7ba00b1d-8cdd-30da-91ad-bb83ed4f7474  ethernet  eth0
usb0       4dab958c-b697-371b-8130-fdefa9c6f529  ethernet  usb0
eth0.4088  cdef5711-7598-314f-9c07-dbc2e438b482  vlan      eth0.4088
lo         f953e31a-389b-452e-b5c0-505609d57444  loopback  lo

# ip -6 addr show usb0
4: usb0: <BROADCAST,MULTICAST,UP,LOWER_UP> ...
    inet6 fe80::2/64 scope link noprefixroute

# ip link show eth0.4088
3: eth0.4088@eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> ...

# ssh root@fe80::1%usb0
root@bmc-oob:~#

# ssh root@fe80::1%eth0.4088
root@bmc-oob:~#