Can you please tell me which PSoC5 device you are using? Please edit the custom linker script according to the flash size of the device. I think the issue might be related to wrong linker script.
Regarding the function, "SelfTest_FlashCheckSum_Init()" you are right, it seems to be missing or not defined or a typo. But since this is not mentioned in the AN, I think it should work without it.
Please attach your project or attach the linker script so that we get more insight into the issue.