Built in QtDC target support in Qt 5.12

Qt for Device Creation commercial offering built in targets.

G1-targets

1. built
2. available through installer (where relevant, assume excluding Win 10 and RTOS targets),
3. documented,
4. tested and verified,
5. part of CI and RTA process (where relevant, assume excluding Win 10 and RTOS targets)

G2-targets

1. partially as G1, there may be limitations or missing bits
2. assumption is that Qt bugs can repro on G1 targets.

For Embedded Linux targets are Yocto based and pre-built images available through Qt Installer and maintenance tool. For other operating systems there may be limitations in e.g. direct availability through installer or e.g. how on-device deployment or debugging can be established.