oe-setup-builddir: Avoid shellcheck warnings

oe-setup-builddir: Avoid shellcheck warnings

This avoid the following warnings:

* SC2086: Double quote to prevent globbing and word splitting.
* SC2164: Use 'cd ... || exit' or 'cd ... || return' in case cd fails.
* SC2166: Prefer [ p ] || [ q ] as [ p -o q ] is not well defined.
* SC2236: Use -n instead of ! -z.

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>