binman: Set a default toolpath

binman: Set a default toolpath

When binman is run from 'make check' it is given a toolpath so that the
latest tools (e.g. mkimage) are used. When run manually with no toolpath,
it relies on the system mkimage. But this may be missing or old.

Make some effort to find the built-from-soruce version by looking in the
current directory and in the builds created by 'make check'.

Signed-off-by: Simon Glass <sjg@chromium.org>