scripts/install-buildtools: add an option to specify where downloads go

scripts/install-buildtools: add an option to specify where downloads go

By default the script puts everything it downloads into a temporary
directory and erases it after unpacking and installing the buildtools.

This isn't great for traceability and reproducibility of builds
(being able to see what was downloaded exactly, and being able
to reproduce setting up a build, especially if the buildtools
download location isn't available for whatever reason).

This commit adds an option to download items into a specified directory
and keep them there. I would particularly like to use it with
bitbake-setup, where an optional feature to install the buildtools
(exact implementation details tbd) would ensure the tarball remains
available on local disk.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>