More external toolchain fixes

Fix issues with binary external toolchains

Fix two problems encountered while using an external binary toolchain
generated by crosstool-ng:

 - Don't remove the ending / in LIB_DIR, otherwise find $LIB_DIR
   -maxdepth 1 doesn't find any file in the case LIB_DIR is a symbolic
   link and not a directory.

   For some reason, find -maxdepth 1 doesn't have the same behaviour
   on directories and symbolic links. Demonstration:

   $ mkdir foobar
   $ touch foobar/t1
   $ touch foobar/t2
   $ ln -s foobar barfoo
   $ find foobar -maxdepth 1 -name 't*'
   foobar/t1
   foobar/t2
   $ find barfoo -maxdepth 1 -name 't*'
   $ find barfoo/ -maxdepth 1 -name 't*'
   barfoo/t1
   barfoo/t2

 * Make sure the libraries are writable, otherwise the strip operation
   might fail. The library files may not be writable if the toolchain
   is not writable (which may happen if one wants to prevent anyone
   from overwriting the toolchain, which is done by crosstool-ng, for
   example).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>