scripts/make_fit: Drop fdt image entry compatible string

According to the FIT image source file format document found in U-boot [1]
and the split-out FIT image specification [2], under "'/images' node" ->
"Conditionally mandatory property", the "compatible" property is described
as "compatible method for loading image", i.e., not the compatible string
embedded in the FDT or used for matching.

Drop the compatible string from the fdt image entry node.

While at it also fix up a typo in the document section of output_dtb.

[1] U-boot source "doc/usage/fit/source_file_format.rst", or on the website:
https://docs.u-boot.org/en/latest/usage/fit/source_file_format.html
[2] https://github.com/open-source-firmware/flat-image-tree/blob/main/source/chapter2-source-file-format.rst

Fixes: 7a23b027ec17 ("arm64: boot: Support Flat Image Tree")
Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

arm64: boot: Support Flat Image Tree

Add a script which produces a Flat Image Tree (FIT), a single file
containing the built kernel and associated devicetree files.
Compression defaults to gzip which gives a good balance of size and
performance.

The files compress from about 86MB to 24MB using this approach.

The FIT can be used by bootloaders which support it, such as U-Boot
and Linuxboot. It permits automatic selection of the correct
devicetree, matching the compatible string of the running board with
the closest compatible string in the FIT. There is no need for
filenames or other workarounds.

Add a 'make image.fit' build target for arm64, as well.

The FIT can be examined using 'dumpimage -l'.

This uses the 'dtbs-list' file but processes only .dtb files, ignoring
the overlay .dtbo files.

This features requires pylibfdt (use 'pip install libfdt'). It also
requires compression utilities for the algorithm being used. Supported
compression options are the same as the Image.xxx files. Use
FIT_COMPRESSION to select an algorithm other than gzip.

While FIT supports a ramdisk / initrd, no attempt is made to support
this here, since it must be built separately from the Linux build.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20240329032836.141899-3-sjg@chromium.org
Signed-off-by: Will Deacon <will@kernel.org>