Another external toolchain support solution

 * In toolchain/external-toolchain/ext-tool.mk, copy the contents of
   the sysroot directory to the staging dir.

 * In package/Makefile.in, add a --sysroot CFLAGS pointing to the
   staging dir

 * Remove the CFLAGS and LDFLAGS definition from
   TARGET_CONFIGURE_OPTS. I haven't investigated exactly why, but with
   these options, DirectFB fails to build because it cannot find
   PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP, even if DirectFB's Makefile
   properly sets -D_GNU_SOURCE.

I have already sent this patch on December, 2nd to the mailing-list,
but got no feedback. So let's commit and see what happens :-)

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