k2hk_evm: add script to automate NAND flash process

Add script to automate NAND flash process. As for now the board has
two burn scripts - burn to boot from SPI NOR flash and burn to boot
from AEMIF NAND flash, rename burn_uboot script to burn_uboot_spi.
Also update README to contain NAND burn U-boot process description.

Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>
Acked-by: Murali Karicheri <m-karicheri2@ti.com>