qtCreatorConfig.sh: fix for use with dora SDKs

In dora the environment-setup script does not set SDKTARGETSYSROOT, only
OECORE_TARGET_SYSROOT. So try to use SDKTARGETSYSROOT first and fallback to
OECORE_TARGET_SYSROOT.
Merge pull request #1 from erbopelagicore/master

qtCreatorConfig.sh: fix for use with dora SDKs