package: add support for top-level parallel make

To be able to use top-level parallel make we must not depend in a rule
on the order of evaluation of the prerequisites, so instead of relying
on the left to right ordering of evaluation of the prerequisites add
an explicit rule to describe the dependencies.

We cannot use the pattern rules because they must have the same
dependency for every package, but we need to change the dependencies
depending on $(2)_OVERRIDE_SRCDIR variable value, so we must use a
more flexible way like $(2)_TARGET_% variables.

So add explicit dependencies for the following stamp files:
  $(2)_TARGET_EXTRACT
  $(2)_TARGET_PATCH
  $(2)_TARGET_CONFIGURE
  $(2)_TARGET_BUILD
  $(2)_TARGET_INSTALL_STAGING
  $(2)_TARGET_INSTALL_TARGET
  $(2)_TARGET_INSTALL_IMAGES
  $(2)_TARGET_INSTALL_HOST

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>