mcf: Add arguments to enable sstate and default premirror

* change premirror behavior that without explicitly set value
  it won't inherit own-mirrors and set url.
* add --enable-default-premirror to set premirror value with
  yocto mirror
* add shorter --premirror variant for --enable-premirror
* add --sstate-mirror to generate SSTATE_MIRRORS in local.conf
  if you want multiple SSTATE_MIRRORS, repeat that option

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <martin.jansa@palm.com>
Change-Id: Ia788452c5169a59829d5650d966982a3ec7b32d1