sourceipk: fix race condition with compileconfigs/multi-kernel

sourceipk: fix race condition with compileconfigs/multi-kernel

* This patch adds an empty compileconfigs function to the
  sourceipk class.
* This is required because when making a sourceipk of a
  package using the multi-kernel.inc functionality you will
  have a race condition between the compileconfigs of the
  multi-kernel file building the kernel and sourceipk trying
  to copy and patch the sources.  The resulting conditions
  can then occur:
    - You will package some interim build files, yielding
      a sourceipk that is not consistent between builds
    - You will try to copy interim build files as they are
      being deleted which will result in an error and break
      the build.
* The empty compileconfigs function is overwritten by the
  real compileconfigs function when the multi-kernel.inc file
  is used.

Signed-off-by: Chase Maupin <Chase.Maupin@ti.com>
Signed-off-by: Denys Dmytriyenko <denis@denix.org>