bootm: update image OS image size when decompressing

In bootm_load_os() the OS image is decompressed. In later stages of the
boot process we need the decompressed size of the image.

Update images->os.image_len after decompression.

Passing the correct size is necessary if we want to check loaded EFI
binararies for file truncation by comparing the loaded size to the header
field SizeOfImage.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>