base-files: make it possible to specify switch led mode

base-files: make it possible to specify switch led mode

The swconfig switch led driver has the ability to switch
between a "link, rx and/or tx" mode. However, this feature
was not implemented in uci, the led init script and
config_generate.

This patch adds a seventh parameter to the
ucidef_set_led_switch() function. The accepted values for
this parameter are: link, rx and tx.
Any permutations of these three values are supported, as
long as they are properly encased with quotes.
If the parameter is not specified it will default to "all"
(link rx tx).

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>