Add CONFIG_NAND_BOOT config flag to organise
NAND_BOOT specific code in config flag like
-nand-boot specfic errata errata_rcw_src()
-CONFIG_SYS_NAND_U_BOOT_DST,etc

Signed-off-by: Santan Kumar <santan.kumar@nxp.com>
Signed-off-by: Priyanka Jain <priyanka.jain@nxp.com>
Signed-off-by: Abhimanyu Saini <abhimanyu.saini@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>