build: image: support specifying mksquashfs4 parallel threads

build: image: support specifying mksquashfs4 parallel threads

By enabling multi-threading file reading, the squashfs rootfs build
process can be greatly accelerated. We use 4 as the default thread
number. This is the default value for squashfs4 tool 4.7 if we don't
append "-block-readers" or "-small-readers" options.

For more test results:
https://github.com/plougher/squashfs-tools/blob/4.7/Documentation/4.7/README

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Link: https://github.com/openwrt/openwrt/pull/19019
Signed-off-by: Nick Hainke <vincent@systemli.org>