kernel-fitimage: dtb sections named by their filenames and one config section for...

kernel-fitimage: dtb sections named by their filenames and one config section for each dtb

Before this, dtb sections were named by their position index in KERNEL_DEVICETREE. Also there was only one item in the config section, so only the first dtb was seen by the bootloader.
This patch adds a config section for each dtb named by the dtb filename. This is what bootloaders usually know about the machine they run on.

(From OE-Core rev: cd2ed7f80b555add07795cc0cbaee866e6c193a3)

Signed-off-by: Florian Wickert <fw@javox-solutions.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>