luci-base: resolve symlinks for PCI hardware

luci-base: resolve symlinks for PCI hardware

getBuiltinEthernetPorts crashed with "Unknown error"
on hardware where NICs have PCI subsystem symlinks at
different sysfs depths (e.g. onboard NIC vs PCIe card NIC).

The getBuiltinEthernetPorts x86_64 workaround block
needs realpath() to resolve PCI subsystem symlinks.

readlink() returns different strings ("../../../bus/pci" vs
"../../../../bus/pci") even though both resolve to /sys/bus/pci,
causing eth1 and eth2 to be incorrectly excluded from the result.

E.g.:

  eth0: /sys/class/net/eth0/device/subsystem -> ../../../bus/pci
  eth1: /sys/class/net/eth1/device/subsystem -> ../../../../bus/pci
  eth2: /sys/class/net/eth2/device/subsystem -> ../../../../bus/pci

1. Add realpath to the 'fs' module import statement
2. Use realpath() to normalize paths

Closes #8425
Reported-by: @MI2-2
Signed-off-by: Paul Donald <redacted>