hob: replace the use of hob-toolchain with populate_sdk task

Now bitbake has an API to run individual task for targets.
Hob can use this to build the sdk, instead of using the hob-
toolchain.

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