oe-setup-build: fix dash support

oe-setup-build: fix dash support

Being minimalist, dash does not support the (non-POSIX) feature of
passing an argument while sourcing a script. Like in
  . <some path>/oe-init-build-env <build dir>

With dash, one must instead use:
  cd <some path>
  set <build dir>       # puts <build dir> in $1
  . ./oe-init-build-env # can only be called from its directory in dash

oe-setup-build generate a sourcable "init-build-env" script, this script
must use the above snippet to be used in dash.

Signed-off-by: Yoann Congal <yoann.congal@smile.fr>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>