Look at available RX-51 commands in README file:
http://git.denx.de/?p=u-boot.git;a=b...nokia_rx51#l45

and their defintions in uboot source code:
http://git.denx.de/?p=u-boot.git;a=b...ia_rx51.h#l281

Basically, to load & book kernel from mmc device (eMMC or SD) card you need:
1) specify mmc device
2) specify mmc partition number
3) specify mmc partition type
4) load kernel file to memory address
5) boot image from memory address

For that set variables mmcnum, mmcpart, mmctype and mmckernfile. And then run trymmckernboot.

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Another option is to connect phone via usb to computer via flasher-3.5 or 0xFFFF load kernel image (in load, not flash) mode and then boot it. This will allows you to boot temporary kernel without flashing it to nand...