oeqa/commands: don't break if get_bb_vars is passed a tuple

oeqa/commands: don't break if get_bb_vars is passed a tuple

get_bb_vars was using variables.copy() to duplicate the list of variables passed
but this function only exists in lists [1,2] and not tuples (1,2).

Instead of throwing an exception if the variables are in a tuple, simply
construct a new list using the passed sequence-like object.

Signed-off-by: Ross Burton <ross.burton@intel.com>