python-config.sh: don't reassign ${prefix}

python-config.sh: don't reassign ${prefix}

When prefix is set to a path like /usr during crossbuild
the sed operations end up executing twice, once for the prefix
reassignment and another for includedir if it is set as a string
including the ${prefix} variable.  This results in an issue
when the build directory is under /usr.

This patch updates the remaining location which uses the prefix
variable to also sed and update to use the real path.

Upstream bug report:
https://bugs.python.org/issue31713

Buildroot bug:
https://bugs.busybox.net/show_bug.cgi?id=10361

Fixes failures like the following:
dbus-python-1.2.4 | NOK | http://autobuild.buildroot.net/results/758858efa97b6273c1b470513f5492258a6d8853

Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
[Arnout: refer to autobuild failures that still exist]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>