CI: label-kernel: support compile testing kernel version and all target

CI: label-kernel: support compile testing kernel version and all target

Add support to label-kernel for compiling testing kernel version and
check patches. To trigger this special build appent :testing to the
normal label.
Example:

- ci:kernel:ipq806x:generic:testing

Test will fail if the requested target doesn't have a defined kernel
testing version.

Also add support for testing all target and subtarget. To trigger this
some special pattern are added:
- ci:kernel:all:all
  Trigger test for all target and subtarget

- ci:kernel:all:first
  Trigger test for all target and the first subtarget in alphabetical
  order for the target.

With these special case :testing can also be used and every target and
subtarget that supports kernel testing version will be selected:
- ci:kernel:all:all:testing
  Trigger test for all target and subtarget that have a kernel testing
  version defined.

- ci:kernel:all:first:testing
  Trigger test for all target and the first subtarget in alphabetical
  order for the target that, if they have a kernel testing version
  defined.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>