I have a compiled linux kernel in the form of uImage for the target device
I also have uboot image for the target device. However, I need an img file that takes the uboot and kernel image and combines them along with the file system to create an image for the device.
Question: Can I manually consolidate uboot and kernel by simply partition the SD card and copy paste kernel image and uboot image, without file system and deploy into the target device? Why do I need a buildroot to do that for me?
The genimage tool does exactly what you need. It is available in Buildroot in the "Host utilities" menu.
Many of the board configurations in the configs directory use genimage to create an SD card image, so they can server as inspiration.