Allow get_bb_var() to work with unexported variable values such as
MACHINE - the workaround is a little crude but should suffice for now.
(From OE-Core rev: 48b58466bba084fd3439706d47e0cfbb7e951ee4)
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>