buildman: Add a way to specific a full toolchain prefix

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 <[email protected]>
Reviewed-by: Joe Hershberger <[email protected]>
Signed-off-by: Simon Glass <[email protected]>
Tested-by: Stephen Warren <[email protected]>