scripts/oe-setup-build: write a build environment initialization one-liner into the...

scripts/oe-setup-build: write a build environment initialization one-liner into the build directory

With this, users no longer have to know where oe-init-build-env is relative to the
build directory; that information is contained in the one liner and then
it's possible to simply use that:

. /path/to/build/init-build-env

This will particularly help with initializing builds in unpacked
build bundles, as users won't have to know where oe-init-build-env
is in the bundle directory tree - similar to esdk initialization.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>