fitImage: add support for multiple dtb and dtbo
initial support for fitImage adds only one primary dtb and conf node. This support enables adding multiple dtb and also adding devicetree overlay (dtbo) nodes.
selecting conf and overlay node can be done with bootm as follows
bootm ${loadaddr}#conf-imx8mn-bsh-smm-s2pro.dtb#conf-imx8mn-bsh-smm-s2pro-display.dtbo
Edited by Parthiban