oe-init-env: search 'bitbake' outside of oe-core

oe-init-env: search 'bitbake' outside of oe-core

atm, 'oe-init-build-env' expects 'bitbake' to be within the OE core git
repository.  This complicates the project setup because you have to
manage the 'bitbake' directory or symlink manually (or specify the
bitbake location explicitly).

Looking for 'bitbake' outside the main git repository will ease project
management significantly.  Now, you can put everything into git submodules,
clone the project with

| git clone --recursive ...

and continue immediately with

| ..../oe-init-build-env

E.g. when you had previously

| .
| |-- build/
| `-- sources/
|     `-- org.openembedded.core/
|         `-- bitbake -> ../bitbake

(where 'bitbake' must be created manually after cloning the project),
you can have now

| .
| |-- build/
| `-- sources/
|     |-- bitbake/
|     `-- org.openembedded.core/

which is completely managed by 'git'.

Patch adds $OEROOT/.. to the search path for 'bitbake'

Signed-off-by: Enrico Scholz <enrico.scholz@sigma-chemnitz.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>