scripts: use script's location to find BSP root directory
scripts: use script's location to find BSP root directory

Implement search for root directory of the Yocto BSP. It uses the folder
".repo" as the anchor.

The BUILDIR is passed as a parameter to oe-init-build-env explicitly,
because the script will otherwise use the current working directory of
the process.

Now the init script should be independent of the caller's current
working directory.

Signed-off-by: Stefan Christ <c04781dbce194802ec6a6311a1200fbcd0141e96@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