freeswitch-stable: bump to 1.6.20

freeswitch-stable: bump to 1.6.20

Upstream seems to have updated their libtool. Instead of running
rebootstrap.sh, set PKG_FIXUP:=autoreconf. Otherwise the build would
fail due to libtool version conflicts.

This is actually nice, because now the build is a bit quicker due to
less autoconf and configure runs.

rebootstrap.sh was primarily used because it worked around some other
issues (e.g. not being able to detect how to convert files to the cross
compiler format, i.e. "func_convert_file_noop"), but that doesn't appear
to be needed anymore, which is nice.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>