icecc: enable the network only when ICECC_DISABLED is not set

icecc: enable the network only when ICECC_DISABLED is not set

Enabling the network uncondictional is not need for some use cases.

Such use case is usefull to reuse the sstate-cache of the build
and it requires the icecc inherit in all of the builds.
The real control control of the icecc is in the variable ICECC_DISABLED
so this patch change the logic to enable the network when the icecc is in use.

Signed-off-by: Jose Quaresma <jose.quaresma@foundries.io>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>