oe-init-build-env: add quotes around variables to prevent word splitting

oe-init-build-env: add quotes around variables to prevent word splitting

Used shellcheck to add quotes to the variables. This is to make sure that
directories with names that have space between, such as
"Desktop/projects/test repo/poky" will not be considered as 2 separate
words. With this modification, running the command "source
oe-init-build-env" will not give the error "bash: oe-init-build-env: No
such file or directory"

Signed-off-by: Abongwa Bonalais Amahnui <abongwabonalais@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>