buildman: allow multiple toolchains in a single path

When buildman scans a toolchain path, it stops at the
first toolchain found. However, a single path can contains
several toolchains, each with its own prefix.

This patch lets buildman scan all toolchains in the path.

Signed-off-by: Albert ARIBAUD <albert.u.boot@aribaud.net>
Acked-by: Simon Glass <sjg@chromium.org>