please do not copy any files to /boot in rootfs because it is the mountpoint for boot-partition and boot will fail if it is not empty. where do you call pkgbuild?
if you do not change filename of kernel (leave uImage and replace the old) you do not need to create a uEnv.txt, only if the name changes or you want to change/extend other vars in the environment. uEnv.txt/uImage needs to be located in the folder you’ve seen above. if you add a new uImage without replacing the old, say uImage_5.3 you create a uEnv.txt and add this at the end (if it exists or you copy an existing file):
kernel=uImage_5.3
btw. instead of copying the 4 files you can simply copy the first 1M from my image to yours…re-read partition table, create filesystems and add kernel+rootfs+modules