buildman: Add a way to specific a full toolchain prefix

At present buildman allows you to specify the directory containing the
toolchain, but not the actual toolchain prefix. If there are multiple
toolchains in a single directory, this can be inconvenient.

Add a new 'toolchain-prefix' setting to the settings file, which allows
the full prefix (or path to the C compiler) to be specified.

Update the documentation to match.

Suggested-by: Stephen Warren <swarren@wwwdotorg.org>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Stephen Warren <swarren@nvidia.com>