econet: switch to new spi/spi-nand drivers

This change produces incompatibility with old flash structure, thus
it's not possible to upgrade old devices (special installation procedure
is required).

There benefits as well:
 * opensource driver
 * more flashes supported
 * better support of spare area
 * more space on the flash is available
 * no special UBI related hack is required
 * no double bad block management

Upgrade procedure:
 0) get new 'u-boot-nand.bin' and 'squashfs.itb' images
 1) backup 'art' partition
 2) rename u-boot-nand.bin image to tcboot.bin and flash it:
    * reboot board and keep it in the bootloader
    * put tcboot.bin to the board using binary tftp node
    * wait for flashing complete and reboot board
    * new bootloader will start
    * execute "run format_ubi" from u-boot command line
    * rename "squashfs.itb" file to "image.itb" and put it to TFTP server on your PC
    * execute "run update_itb" twice from u-boot command line
    * execute "setenv board_id rodimus && saveenv" from u-boot command line
    * reboot board, new image will be booted.
 3) restore 'art' partition

Downgrade procedure:
 0) get old bootloader and 'squashfs.itb' images
 1) rename old bootloader to "uboot-nand.bin" and put it to TFTP server on your PC
 2) reboot board and keep it in the bootloader
 3) execute "run update_uboot" from u-boot command line
 4) wait for flashing complete and reboot board to start old bootloader
 5) flash old images and recover art partition