[oe] meta-qt5: dependency loop for egl-library build by qmake

[Andreas Müller]

Hi,

Found a libegl which is build by qmake. This turns into a dependency loop:

* qtbase ships mkspec and depends on egl (if enabled by packageconfig)
* the driver needs mkspec to build

I see two possible way to get out:

1. in qmake5.bbclass set 'OE_QMAKE_PLATFORM ?= "linux-oe-g++"' Hereby
the driver can ship and select a copy of linux-oe-g++ with a different
name. Disadvantage: in case something is changed on mkspec the copy
needs alignment.
2. Split out mkspec into a recipe. qmake5.bbclass can then depend on
that recipe. Disadvantage: in case linux-oe-g++ finds it's way to
mainline this won't work no more.

What do you think?

Andreas