tools/autoconf: Fix relocatable patch

tools/autoconf: Fix relocatable patch

Fix the 000-relocatable.patch broken by e0f5ce9. The patch segment about
detecting STAGING_DIR_HOST was erroneously removed, as upstream had
deleted the previous bin/autoconf.as and had implemented it in perl
in bin/autoconf.in. Re-create the previous functionality in that.

Fixes: #18059
Fixes: e0f5ce974 ("tools/autoconf: bump to 2.72")
Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>
Link: https://github.com/openwrt/openwrt/pull/18073
Signed-off-by: Robert Marko <robimarko@gmail.com>