The tool/prepare_port tool is now able to handle a list of ports that
shall be prepared. Additionally, one may state the number of ports that
shall be prepared in parallel at a max by using the -j parameter. If -j
is not set by the user, the tool acts as with -j1. The previous
implementation of the tool that prepares only a single port was moved to
tool/ports/mk/prepare_single_port.mk and acts as back end to the new
prepare_port tool. The interface of the new prepare_port tool is
backwards compatible. When called for one port only, the behavior is the
same as when calling tool/ports/mk/prepare_single_port.mk directly.
Removes "usage" rule from prepare_single_port.mk. Removes shebang line
from prepare_single_port.mk.
Ref #1872
