scripts/install-buildtools: Handle new format checksum files

Autobuilder generated checksum files only have a single space between the sum and the
filename, tweak it to account for this.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit d1e71bf5)
Signed-off-by: Steve Sakoman <steve@sakoman.com>