buildman: Add an option to use the full tool chain path

In some cases there may be multiple toolchains with the same name in the
path. Provide an option to use the full path in the CROSS_COMPILE
environment variable.

Note: Wolfgang mentioned that this is dangerous since in some cases there
may be other tools on the path that are needed. So this is set up as an
option, not the default. I will need test confirmation (i.e. that this
commit fixes a real problem) before merging it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: Steve Rae <srae@broadcom.com>