buildman: Introduce an 'and' operator for board selection

Currently buildman allows a list of boards to build to be specified on the
command line. The list can include specific board names, architecture, SOC
and so on.

At present the list of boards is dealt with in an 'OR' fashion, and there
is no way to specify something like 'arm & freescale', meaning boards with
ARM architecture but only those made by Freescale. This would exclude the
PowerPC boards made by Freescale.

Support an '&' operator on the command line to permit this. Ensure that
arguments can be specified in a single string to permit easy shell quoting.

Suggested-by: York Sun <yorksun@freescale.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: York Sun <yorksun@freescale.com>