imxtract: specify max gunzip size

Specify max gunzip size from config to override SYS_XIMG_LEN
default value wich is 0x800000. In case we have a large portion of FIT
image, for example gzipped kernel with decompressed size large than
0x800000 we should enlarge imxract area, otherwise extracting it will
fail.

It used to be a hardcoded define in cmd/ximg.c and we are moving it to
config.

Signed-off-by: Nikita Shubin <n.shubin@yadro.com>
Reviewed-by: Simon Glass <sjg@chromium.org>