scripts/bitbake: allow switching between build directories

scripts/bitbake: allow switching between build directories

The recent addition of the check to ensure the user was in their build
directory disabled the ability to switch between build directories
without re-running the build environment setup script. We can rely
upon checking for conf/bblayers.conf instead, so use this check.

This does allow BUILDDIR (which is normally set by the environment
script) to be unset; however if it is set then it is assumed to be the
correct build directory and will be used in the error message that is
shown when we can't find conf/bblayers.conf.

(From OE-Core rev: 769384decb095fb3c49eb13b8f7f69c978d0bcba)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>