Add option to oe-buildenv-internal script to change bitbake location.

Add option to oe-buildenv-internal script to change bitbake location.

Having bitbake inside the oe-core is annoying to some people. This commit
adds a second option to the oe-init-build-env script.

Run like this:

. ./oe-init-build-env ../build ../bitbake

for example. Without the second option, the old behavior is preserved.

Signed-off-by: Philip Balister <philip@balister.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>