sunxi-leds: Add optional leds_default_# and leds_inverted_# fex parameters

leds_inverted_# is useful for active_low leds obviously :)
leds_default_# is useful for powerleds which are connected to gpio pins, to
make them light up on boot automatically (and stay on as long as they are not
re-purposed)

Note that rather then introducing yet more variables to store the key
to search the leds_para section by, this patch uses a single key string
buffer to store this, and to make this work it slightly re-arranges the order
in which things are read from the fex file.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>