Makefile: Fix "make clean"

This fixes a dangerous bug: "make clean" after "make distclean" will
delete every single file including those under .git, if you do in-tree
build!

Rationale: A first "make distclean" will unset $(DSOSUF), a following
"make distclean" or "make clean" will find all the files and delete it.

Fix it by explicitly typing the file extensions here, and combine
multiple find invocations into one.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-id: 1395020122-4957-1-git-send-email-famz@redhat.com
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>