cmd: zip: automatically pull in gzip()

Move the CONFIG_GZIP_COMPRESSED from a config.h macro to a Kconfig menu
item. It is not selectable by a user because there is no reason to do
so. Instead it will be automatically selected by the stuff which uses
gzip(), like the zip command.

Remove it from the config_whitelist.txt. Also remove
CONFIG_GZIP_COMPRESS_DEF_SZ as this was never used on any board. The
default seems to be sane, otherwise it should be added as a Kconfig
option.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Simon Glass <sjg@chromium.org>