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.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
bin/bitbake: Improve --help text

The --help text was rather inconsistent in style and plain incorrect in places,
using confusing terminology in others. I guess most people know what the options
do and don't read this but its confusing to new users.

This updates it to use the terms recipe and task consistently, remove
the references to stage, bbread and generally try and make the output
more useful.

[YOCTO #4856]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
hob: populate "Save image" dialog when saving changes to a custom image recipe

If my build starts from a YP standard image recipe, the 'Save image
recipe' dialog should be empty.

If my build starts from a custom image recipe, the 'Save image recipe'
dialog should populate the 'Name' and 'Description' fields with the values
set for the custom image recipe.

[YOCTO #5004]
Signed-off-by: Cristiana Voicu <cristiana.voicu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>