Is there somewhere documented which configuration parameters can be used with U-Boot from the Apalis TK1 Linux Image (BSP) v2.7b1+?
Please note that that beta BSP is no longer supported.
(Are the CONFIG_… parameters in u-boot/configs/apalis-tk1_defconfig which are commented out with # the only parameters which can be used in addition to the ones already configured to Y?
No, commenting out parameters in a defconfig serve a completely different purpose. This actually disables a certain feature which would otherwise be enabled by default.
Or are there other config values which are not commented out there?)
I don’t think there exist a concluding such list. But for such U-Boot specific questions you may be better off asking on the U-Boot mailing list. Be prepared to switch to using mainline U-Boot thought.