package/pkg-generic: add the concept of extract dependency

package/pkg-generic: add the concept of extract dependency

Extract dependencies are dependencies that must be ready before the
extract step of a package, i.e for tools that are needed to extract
packages themselves. Current examples of such tools are host-tar,
host-lzip and host-xz.

They are currently handled through DEPENDENCIES_HOST_PREREQ. However,
this mechanism has a number of drawbacks:

 - First and foremost, because host-tar/host-lzip/host-xz are not
   listed in the dependencies of packages, the package infrastructure
   does not know it should rsync them in the context of per-package
   SDK.

 - Second, there is no dependency handling *between* them. I.e, we
   have no mechanism that says host-tar should be built before
   host-lzip, while it is in fact the case: if you need to build
   host-lzip, you need to extract a tarball, so you may need host-tar
   if your system tarball is not capable enough.

For those reasons, it makes sense to add explicit support for "extract
dependencies" in the package infrastructure, through the
<pkg>_EXTRACT_DEPENDENCIES variable. It is unlikely this variable will
ever be used by a package .mk file, but it will be used internally by
the package infrastructure.

[Peter: fix typo in manual]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>