Use 'sysroot' rather than 'sys-root' for the sysroot/staging_dir

As pointed out on the list, using sysroot rather than sys-root is less
confusing, as this is how it is referred to in the GCC manual.

So rather than changing BR, patch ct-ng to use sysroot instead.
The next ct-ng release will use 'sysroot' as well by default.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>