oe-setup-builddir: small rework

oe-setup-builddir: small rework

Changes:
- drop useless subshell creation in test:
if ! (test -r "$BUILDDIR/conf/local.conf"); then$
- replace "source" builtin by "." (bashsism)
- fix indentation 4 spaces (drop some tabs too)
- fix return => exit  (return is not allowed in main)
- drop "sed -i" (doesn't exist in BSD sed)
- for homogeneity, always use [ ] (instead of test)
- replace old [ "x" = "x$VAR" ] by [ -z "$VAR" ]

(From OE-Core rev: 3a116577446f02bda0ef4e035360293ff73c9eef)

Signed-off-by: Matthieu Crapet <Matthieu.Crapet@ingenico.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>