BuildImages: fix mystery whitespace in build logs

In some forms the "Building image1 image2" message is interleaved with
whitespace.  This is because the description field is meant to be a list but
this class sets it as a string.  Thanks to duck typing the list operations to
join it with " " work fine, and the message ends up spaced.  Fix this by setting
the description as a list.

Signed-off-by: Ross Burton <ross.burton@intel.com>
BuildImages: don't override describe and getText

BuildStep.getText() will call describe() so just put the extra logic in there.
Also remove some trailing whitespace.

Signed-off-by: Ross Burton <ross.burton@intel.com>
RunPreamble: remove pointless code

Remove unused imports, trailing whitespace, local variables that are not used, a
timeout change that isn't needed, and a pointlessly overriden method.

Signed-off-by: Ross Burton <ross.burton@intel.com>