build: Only first-level subdirectories with `mk_sub_build_dir`

build: Only first-level subdirectories with `mk_sub_build_dir`

We have two pairs of functions that are internally used to create the
directory structure the build output directory (normally `build/`):
- The function `mk_sub_build_dir` is intended to create only the first
  level subdirectory under the build output directory for a library.
  `sub_build_dir` is a companion function that returns the full path
  of such a first-level subdirectory for a given library name.
- The function `mk_sub_libbuild_dir` is intended to create further
  subdirectories under a library build directory that was created by
  `mk_sub_build_dir`. This function allows multiple levels of
  subdirectories (example: to create an header include folder:
  'include/uk/bits').
  `sub_libbuild_dir` is the companion function that returns the
  full path of such a library subdirectory.

This commit corrects the implementation so that this behavior is
enforced.

Signed-off-by: Simon Kuenzer <simon@unikraft.io>
Reviewed-by: Michalis Pappas <michalis@unikraft.io>
Reviewed-by: Robert Kuban <robert.kuban@opensynergy.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #938