mklibs: Fix mklibs usage when *.so files reside in subdirs

Currently package/Makefile only passes /lib /usr/lib and /usr/lib/ebtables
to mklibs. However, other libs can also reside in different subdirectories
(in my case /usr/lib/ipsec). Hence, create the list of library directories
dynamically.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37841 3c298f89-4303-0410-b956-a3cf2f4a3e73