scripts: on startup, also remove the buildtools dir

In case there's one lingering around (whether the previous build was
successful, or failed), we have to remove the buildtools directory
as well as the toochain build dir.

This should also fix the case where out makeinfo wrapper calls
itself recursively.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
(transplanted from 0e8ff57073838afea8830fe09d1d01cdf7cb37d8)

 COPYING