[oe] making qt5 package machine specific

[Nicolas Dechesne]

hi,

we are building a product/distro with qt5 with support for various SoC. the
way qt5 recipes are designed, it kind of forces the resulting packages to
be  <machine> specific, not <arch> specific.

first each <machine> has a different provider for GL/GLES and it impacts
the RDEPENDS, on top of that we have machine specific patches in Qt5 in
each of our BSP layer, whether we like it or not, that's a reality that we
cannot have the very same Qt5 config + source code for all platforms.

so we end up with many  <qt pkgs>-<arch> which are <machine> specific, not
<arch>, and it messes up badly with sstate for example, especially when
cleaning up the sstate (e.g. using sstate-cache-management -d).

so i have a couple of questions:

 - are we doing something really wrong here? or are we getting issues that
anyone would get when trying to have a OE based product with Qt5 and
multiple machines from the same <arch>?

 - assuming this is a typical use case... i wish there was an easy
mechanism to 'mark' all Qt5 packages as PACKAGE_ARCH = "${MACHINE_ARCH}",
without having to .bbappend every single recipe. It is indeed quite common
to have a .bbappend for qtbase, but not for the other packages...

thanks!