patman: Add a function to find a tool on the path

patman: Add a function to find a tool on the path

The Run() function automatically uses the PATH variable to locate a tool
when running it. Add a function that does this manually, so we don't have
to run a tool to find out if it is present.

This is needed by the new Bintool class, which wants to check which tools
are present.

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