bitbake.conf: Add SDKPKGSUFFIX to hash whitelist

The gcc recipes reference this however we account for it in the work
directory paths and we don't want recipes depending on the value changing.
This avoids unecessary rebuilds when switching SDKs.

(From OE-Core rev: 6cdcc543ce8f532a4f66246114241b43821a111e)

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