[OE-core] what is the precise search path for resolving "include" or "require" .inc files?

[Robert P. J. Day]

   not sure why i never noticed this before, but what is the search order
or precedence or priority for resolving recipe file directives "include"
or "require"? i was just perusing the bitbake user manual, and noticed
the explanation that those directives use BBPATH to track down the first
include file by a given name.

   but AIUI, BBPATH is supposed to represent an ordered set of *layers*
(i just examined BBPATH for a current [wind river linux] build, and that
is exactly what it seems to contain). so when a recipe file like
busybox.whatever.bb contains the line:

   require busybox.inc

i always just assumed the search would look in the current busybox
directory, but that directory is not technically a layer directory --
it's not a directory specified on the alleged BBPATH search path.

   i never thought about this until i noticed some .bbappend files
specifically using directives like:

   require recipes-bsp/u-boot/u-boot.inc

where that file reference *does* refer to a file relative to some
actual layer directory somewhere.

   what am i missing here?

rday