Sometimes you may want to store some data in compressed format (for example to save flash memory), while your application needs the data in uncompressed form. You can trick U-Boot to do this uncompressing like this:

Generate a compressed U-Boot image of type "standalone" (="mkimage ... -T standalone -C gzip ..."=) and make sure that the envrionment variable autostart is set to no (i. e. enter "setenv autostart no;saveenv"). If you then use "bootm" for such an image, U-Boot will uncompress the contents of the image and store it at the "load address" ("-a" option for mkimage), but not attempt to start it yet. If the image contains executable code, you can omit the setting of "autostart", and U-Boot will automagically start the image by jumping to the entry point address ("-e" option for mkimage).