OpenEmbeddded Overlay

               
在gentoo dev manual上Overlay这一章中写道：
Portage can look in multiple places for packages by using an overlay. The
locations of overlays are controlled by the PORTDIR_OVERLAY variable, which
should contain a space-separated list of paths.
The overlay should contain the same directory structure as PORTDIR (although
only the necessary directories need be included). For example, a simple overlay
might have a directory structure like:
               
               
                overlay
|-- dev-util
    `-- gengetopt
        |-- Manifest
        |-- files
        |   `-- digest-gengetopt-2.13
        `-- gengetopt-2.13.ebuild
OpenEmbedded源自gentoo，Overlay这个特征也继承了。
例如：
BBFILES = "/stuff/openembedded/*/*.bb /stuff/openembedded.modified/*/*.bb"
// 所有的bbfile
BBFILE_COLLECTIONS = "upstream local"
// 所有的开发分支
BBFILE_PATTERN_upstream = "^/stuff/openembedded/"
BBFILE_PATTERN_local = "^/stuff/openembedded.modified/"
// 分支的正则表达式
BBFILE_PRIORITY_upstream = "5"
BBFILE_PRIORITY_local = "10"
// 分支的优先级
那么如果在local和upstream同时拥有一个demo package,那么bitbake就会选取优先级高的bbfile，优先级
就是根据
BBFILE_PRIORITY，数值越大的，优先级越高
也就是我们可以不修改upstream中的pakcage,直接在local里添加修改新的package,拥有自己的开发分支