The reason that the upgrade planner shows 0 for minimum memory and flash, is because bootloader images will all run with even the least possible amount of memory and flash. Boot loader images are extremely limited in functionality, and are simply meant to help load the IOS image as well as a fallback mechanism if the IOS image is missing or corrupt so that you can at least have IP connectivity to a TFTP server to load a valid image.
Also, it is generally a good idea to have a boot image match the system image, but it's not absolutely necessary by any means.
HTH,
Bobby
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