SDKtool: Fix test case and compiler warning

Do check for toolchains not defined in the toolchains.xml file again,
but also allow for toolchain IDs to be ABIs.

This partially reverts a commit from Kai made earlier.

Change-Id: Idad004f0bff655bc136bc43de5837d61a045e311
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>