ArchLinux packages need working symlinks in $srcdir

fixes https://github.com/openSUSE/open-build-service/issues/658

problem:

  `makepkg -o` does (with `PWD=$TOPDIR/SOURCES`):

    mkdir src
    for s in ${source[@]}; do
      ln -s $SRCDEST/$s src/
    done

  where `$SRCDEST` is `$PWD` unless overridden.

  then we move `$TOPDIR/SOURCES/*` (`s/SOURCES/BUILD/`), and the
  symlinks still contain the old paths.

solution:

  run `makepkg -o` in `$TOPDIR/BUILD`.
fix makepkg failing to find install, changelog

parent breaks building packages with install script and/or
changelog file.  these files "should reside in the same directory
as the PKGBUILD" according to PKGBUILD(5), but makepkg code handling
them fails unless PKGBUILD is in the $PWD.
fix placement and recovery of the build product

makepkg puts the build product in ${PKGDEST-$PWD}, PKGDEST was not
set, and PWD=$TOPDIR/SOURCES, but recipe_build_arch() was looking
in $TOPDIR/BUILD.

half of recipe_build_arch() is replaced by PKGDEST=$TOPDIR/ARCHPKGS.
makepkg commandline shorter with --config ...

it's easier to read and tweak this way.
Merge pull request #105 from roman-neuhauser/arch-src-links

ArchLinux packages need working symlinks in $srcdir