Add package managers as a plugin

["Fredrik Gustafsson" <fredrik.gustafsson@axis.com>]

OE-core today has three different package managers, the well-known formats deb
and rpm is supported as well as ipkg that is good for embedded devices.

When building and having a good cache hit, a significant amount of time is
spent in the phase of generating a rootfs, which is really about the
performance of the package manager. To save build time and also get a
package manager that is suitanle for use on targets where flash memory is a
concern, support for apk is suggested.

However, it might or might not be what's wanted for OE-core since it increases
the test matrix. Therefore I will refactor the package management code to
allow a layer to add a new package manager without editing the meta layer.

This refactor will be divided into multiple patch series to be easier to review,
this first one with a single patch.