combo-layer: introduce ability to exclude component from mass update
There is no ability at the moment for situations where users would like
to keep section in combo-layer.conf but don't update it, unless explicitly
specified.
Now, by adding "update = no" to desired section would exclude
that repository from "combo-layer update" command.
It is still possible to explicitly update it by "combo-layer update $section".
By default, all repositories are assumed as "update = yes"
Signed-off-by: Alexander D. Kanevskiy <kad@kad.name> Signed-off-by: Ross Burton <ross.burton@intel.com>
