common/Makefile: don't include env_embedded.o into libcommon

Some boards use an embedded environment, where env_embedded.o has to
be linked at a special position in the U-Boot image; to make this
possible, we do not include it into libcommon.o for such boards.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Wolfgang Denk <wd@denx.de>
Acked-by: Stefan Roese <sr@denx.de>