Makefile.uk: Use mk_sub_libbuild_dir to create directory hierarchies

Makefile.uk: Use mk_sub_libbuild_dir to create directory hierarchies

Unikraft core build system currently enforces the use of
`mk_sub_libbuild_dir` function instead of `mk_sub_build_dir` for
creating directory hierarchies, i.e. creating more than the first-level
subdirectory.

Update `Makefile.uk` accordingly: replace the call to `mk_sub_build_dir`
with a call to `mk_sub_libbuild_dir`.

Signed-off-by: Razvan Deaconescu <razvand@unikraft.io>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Approved-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #9