scripts: init_machine: add config option
scripts: init_machine: add config option

Setting a MACHINE can now be done by setting the environment variable
MACHINE before calling phyLinux or tools/init. The priority is now:

 1. shell env "MACHINE"
 2. manifest.xml
 3. selection menu

Signed-off-by: Stefan Müller-Klieser <a3e08e35e4a42be768e7d0e76ae1ba321c2a66dd@phytec.de>

PHYTEC-Messtechnik-GmbH/meta-phytec,PHYTEC-Messtechnik-GmbH/meta-phytec,PHYTEC-Messtechnik-GmbH/meta-phytec,PHYTEC-Messtechnik-GmbH/meta-phytec,PHYTEC-Messtechnik-GmbH/meta-phytec