The existing zstd API requires the same sequence of calls to perform its
task. Create a helper for U-Boot, to avoid code duplication, as is done
with other compression algorithms. Make use of of this from the image
code.
Note that the zstd code lacks a test in test/compression.c and this should
be added by the maintainer.
Signed-off-by: Simon Glass <sjg@chromium.org>
