Add mkdtimg: a tool to create dtb/dtbo images

mkdtimg is a tool to create DT images with one ore more DTB files
(.dtb). The output images can be used for dtb/dtbo partitions.

It will generate a header with dt_table_header and dt_table_entry, and
append all DTB files into image. It can also dump informations in
an DT image file.

Bug: 35176347
Test: run mkdtimg_testdata.sh
Change-Id: I3f17a1c340e23b734af63a30e04d384fc9cbfa3c