[FIT] support multiple DTB blob inside FIT
Rockchip scripts for FIT node generation are used to generate u-boot.its; this file is then used to generate u-boot FIT image. When using CONFIG_MULTI_DTB_FIT we need to insert multiple DTB blob (fit-dtb.blob) in the place of u-boot.dtb. This blob is again a FIT image, containing all the possible DTBs for the board, listed in CONFIG_OF_LIST.