apr-util: fix the rules.mk with path to sysroot instead of the workdir of apr

apr-util: fix the rules.mk with path to sysroot instead of the workdir of apr

The copying for rules.mk was happened in do_configure_prepend, but it will be
replaced by the one generated by configure, in which APR_MKEXPORT points to
the workdir of apr and cause compile filure when the workdir of apr is removed.
So change the copying in _prepend to _append.

[YOCTO #2947]

(From OE-Core rev: d6aa123fdbfa82c07e33a9b4c7aacad821184790)

Signed-off-by: Jackie Huang <jackie.huang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>