package_ipk: allow to specify OPKG_ARGS in local.conf

package_ipk: allow to specify OPKG_ARGS in local.conf

If user specific parameters to opkg are set in local.conf, they are
rewritten in package_ipk.bbclass and ignored, instead append
package_ipk specific arguments to the user defined ones.

The change is needed, if a user has to pass an alternative path to a
temporary directory for opkg, e.g.

  OPKG_ARGS = "--tmp-dir=${TOPDIR}/tmp-opkg"

The default /tmp directory may be unusable for do_rootfs task, for
example if there is no enough space or /tmp is mounted with noexec
mount option, then an alternative path allows to complete do_rootfs
and fix the problems like this:

  ERROR: Unable to install packages.
  ...
  sh: /tmp/opkg-5jPLag/run-postinsts-UsUtaI/preinst: /bin/sh: bad interpreter: Permission denied
  sh: /tmp/opkg-5jPLag/base-files-4hFwQS/preinst: /bin/sh: bad interpreter: Permission denied
  sh: /tmp/opkg-5jPLag/run-postinsts-UsUtaI/preinst: /bin/sh: bad interpreter: Permission denied
  sh: /tmp/opkg-5jPLag/busybox-syslog-sJmfbw/preinst: /bin/sh: bad interpreter: Permission denied
  ...

(From OE-Core rev: 0be4c30aaf03de58d8037a89e342ca0b379a0881)

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>