base-files: add led functions to uci-defaults.sh

base-files: add led functions to uci-defaults.sh

uci_set_leds_ataport() allows to set a led to show activity
on a specific (s)ata port, which is needed for devices that have
a Sata led for each sata port.
The led trigger is from the 834-ledtrig-libata.patch LEDE kernel patch.

uci_set_leds_usbhost() allows to set a led to show total usb activity.

Signed-off-by: Alberto Bursi <alberto.bursi@outlook.it>
[Jo-Philipp Wich: use a single underscore to denote private functions]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>