Implement ??= operator

Implement ??= operator

??= is a lazy, conditional assignment.  Whereas a ?= immediately assigns to
the variable if the variable has not yet been set, ??= does not apply the
default assignment until the end of the parse.  As a result, the final ??= for
a given variable is used, as opposed to the first as in ?=.

Note that the initial implementation relies upon finalise() to apply the
defaults, so a "bitbake -e" without specifying a recipe will not show the
defaults as set by ??=.  Moving application of the default into getVar adds
too large a performance hit.  We may want to revisit this later.

Signed-off-by: Chris Larson <chris_larson@mentor.com>