oe-init-build-env: create a symlink in BUILDDIR

oe-init-build-env: create a symlink in BUILDDIR

When there are multiple oe-core repos (for example, poky-master,
poky-morty and poky-krogoth) and multiple builddirs for them, it's not
easy to reinit builddir in such a case, and very easy to make mistakes.
Create a symlink in builddir will make it easy, for example:

e.g.:
$ . poky-morty/oe-init-build-env test_m
$ ls -l oe-init-build-env
oe-init-build-env -> /buildarea/lyang1/poky-morty/oe-init-build-env

When we want to reinit the builddir test_m:
$ . oe-init-build-env .

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>