Stop duplicating ${myconf}

Doing myconf+="${myconf} ..." in the ebuild caused us to append not only
our addition to ${myconfig}, but also to prepend the contents of
${myconf} to itself. This resulted in configure script lines that looked
something like this:

./configure ... --with-spl="/usr/src/spl-0.6.1" --with-spl="/usr/src/spl-0.6.1"
--with-zfs="/usr/src/zfs-0.6.1" ...

We switch to myconf="${myconf} ..." to correct that.

Signed-off-by: Richard Yao <ryao@gentoo.org>