[poky] Extending meta-toolchain-sdk

[Cui, Dexuan]

Mike Detwiler wrote:
> I tried the following line instead:
> 
> SDK-EXTRAS += "gmp-dev"
> 
> This did not produce an error, but it also did not seem to add the
> library to the sdk. Is it possible that this is because the variable
> SDK-EXTRAS is also set in meta/recipes-gnome/tasks/task-sdk-gmae.inc
> with the line below.
> 
> SDK-EXTRAS = ""
> It looks like this could clobber my setting of the variable, depending
> on the order of evaluation.
I think we can change the line of task-sdk-gmae.inc to
SDK-EXTRAS ?= ""
This allows the setting of environment variable to be used if a user does set it.
And I think increasing the PR of the related recipe files is also necessary, or else, the change may not actually take effect, unless we build from scratch or do a "bitbake -c clean xxx".

Thanks,
-- Dexuan