[OE-core] what is the value of 'BBPATH ?= ""' in layer.conf?

[Mark Hatle]

On 11/22/15 3:10 AM, Robert P. J. Day wrote:
> 
>   perusing some wind river layers, and in the layer.conf file here:
> 
> https://github.com/WindRiver-OpenSourceLabs/wr-kernel/blob/LB21_7.0_RCPL0002/conf/layer.conf
> 
> i see the opening snippet:
> 
>   BBPATH ?= ""
>   # We have a conf and classes directory, add to BBPATH
>   BBPATH := "${LAYERDIR}:${BBPATH}"

(for the -first- layer defined)

BBPATH := "${LAYERDIR}:${BBPATH}"

will result in a warning or error because the ${BBPATH} can't be expanded.

There was a commit late 2013 to OE-Core and most related layers that turned this
into a append (or in this case prepend):

BBPATH =. "${LAYERDIR}:"

This is stop the need for the BBPATH and work properly.

LAYERDIR is always expanded when used, so no need for the :=

--Mark

> what is the purpose of that initial assignment
> 
>   BBPATH ?= ""
> 
> if that variable has no value, won't it just expand to the empty
> string so that the eventual effect is the same? or is there something
> more subtle happening here?
> 
> rday
>