libpayload: By default build libflashrom.a instead of flashrom.

flashrom won't build nor run as native payload very soon (or ever).
This patch changes a special GNU make variable that allows to select
the default goal which is taken if no goal is given explicitly on
the command line. Normally this would be the first rule in Makefile,
i.e. all. This won't compile if the target OS is libpayload, hence
change it to "libflashrom.a" in that case.

This requires two not completely ancient GNU make features:
 - MAKECMDGOALS
 - .DEFAULT_GOAL

Checking for these with ancient-only GNU make features is non-trivial
and hereby postponed.

Signed-off-by: Stefan Tauner <stefan.tauner@student.tuwien.ac.at>
Acked-by: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>

git-svn-id: svn://flashrom.org/flashrom/trunk@1726 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1