bitbake: build: Add BB_TASK_NICE_LEVEL to task code

bitbake: build: Add BB_TASK_NICE_LEVEL to task code

On Linux its not possible for processes to regain a previous nice level after
it has changed. Its therefore not possible to have a core low priority and
then raise the priorities of individual tasks.

This variable allows us to do something like:

BB_TASK_NICE_LEVEL = "5"
BB_TASK_NICE_LEVEL_task-testimage = "0"

to give priority to specific tasks which the BB_NICE_LEVEL functionality
doesn't give us the option of.

(Bitbake rev: 94d82997220c6cfc7028f76719df028ba8254a5c)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>