utils: reimplement/fix mkdir_p()

Reimplement mkdir_p() such that it:
 ...handles relativ paths correctly. (currently it crashes)
 ...does not rely on dirname().
 ...is not recursive.
 ...is shorter. ;-)

Signed-off-by: Richard Weinberger <richard@nod.at>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>