scripts/make_fit: Support an initial ramdisk

scripts/make_fit: Support an initial ramdisk

FIT (Flat Image Tree) allows a ramdisk to be included in each
configuration. Add support for this to the script.

This feature is not available via 'make image.fit' since the ramdisk
likely needs to be built separately anyway, e.g. using modules from
the kernel build. Future work may provide support for doing that.

Note that the uncompressed size is not correct when a ramdisk is used,
since it is too expensive to decompress the ramdisk.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Reviewed-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Link: https://patch.msgid.link/20260106162738.2605574-3-sjg@chromium.org
Signed-off-by: Nathan Chancellor <nathan@kernel.org>