As ${OE_QMAKE_CXX} is empty, that indicates to me that you haven't source environment-setup-cortexa9hf-vfp-neon-poky-linux-gnueabiasenvironment-setup-cortexa9hf-vfp-neon-poky-linux-gnueabi.

This should work to compile by hand:

 . /opt/poky/charles/environment-setup-cortexa9hf-vfp-neon-poky-linux-gnueabi
 echo $OE_QMAKE_CXX
 qmake
 make

(Assuming you don't need to pass any extra arguments to qmake).

Then do the following:

 . /opt/poky/charles/environment-setup-cortexa9hf-vfp-neon-poky-linux-gnueabi
 echo $OE_QMAKE_CXX
 qtcreator

Note that you should start qtcreator from the same shell that you source environment-setup-cortexa9hf-vfp-neon-poky-linux-gnueabi in.

This should get you going. Otherwise I'd suggest starting by getting a small, simple Qt appliction building correctly.

For more in configuring QtCreator, you could for instance have a look at how to setup QtCreator for cross-compilation.

Update

As it seems that Qt5 isn't included in the SDK at all, we first have to generate a suitable SDK. My preferred way:

 bitbake your-image -c populate_sdk

This works, as long as your image recipe includes

 inherit populate_sdk_qt5

That would give you an SDK, whose sysroot would match your image.

The "older" way, is to use a special toolchain recipe. For Qt5 that would be meta-toolchain-qt5, or some recipe that includes/requires that one. In this case, you would run:

  bitbake meta-toolchain-qt5

Though, the recommended way is to use the image specific SDK.