Replace os.makedirs() with 'mkdir -p'

Resolves issue with concurrent workers creating directories. E.g.,

    File "PKGTOOLS/cmsBuild", line 2805, in buildPackage
    makedirs (pkg.builddir)
    File "/usr/lib64/python2.4/os.py", line 156, in makedirs
    makedirs(head, mode)
    File "/usr/lib64/python2.4/os.py", line 156, in makedirs
    makedirs(head, mode)
    File "/usr/lib64/python2.4/os.py", line 159, in makedirs
    mkdir(name, mode)
    OSError: [Errno 17] File exists: '/<snip>/a/BUILD/slc5_amd64_gcc472/external'

Signed-off-by: David Abdurachmanov <david.abdurachmanov@gmail.com>
Signed-off-by: Giulio Eulisse <giulio.eulisse@gmail.com>