The yocto project allows the use of relative path in most of its configuration files but not within the ./build/conf/bblayers.conf file. What is the reason for blocking the use of anything but absolute paths for the BBLAYERS and BBLAYERS_NON_REMOVABLE variables?
I have looked at the BitBake user manual for yocto version 2.0 (current release) but that does not explain the reasoning. I also checked some of the older manual versions but they do not seem to mention the reasoning when talking of the bblayers.conf file or the BBLAYERS variable. The same file also contains BBPATH = "${TOPDIR}" which is at least dynamically assigned and not that far away from the root yotco directory.
My best guess is that the bblayers.conf file is specific to the system it is being run on. That would make it unsuitable for sharing between developers via source control and the absolute paths would force people to edit the file whenever they received a copy. That did not seem like a very good reason though, hence the question.
All the existing answers are answering "how to use relative paths" but the question is "why are absolute paths used". As far as I know the "why" is simple, it is done that way so that the build directory can be moved anywhere on the filesystem. Think about it: you can source poky/oe-init-build-env from anywhere on the filesystem and the build directory will be created there, so relying on paths relative to the build directory is very fragile.
Edit:
maybe this is clearer, I think you are assuming that the file bblayers.conf is always in poky/build/conf/bblayers.conf and that you can therefore use a path like ../../meta-layer-foo to refer so some layer which would be in poky/meta-layer-foo, but the layer will not be found if I instantiate "build" in another path poky/foo/bar:
etienne@ubuntu:~/repos/poky-tx2$ mkdir -p foo/bar
etienne@ubuntu:~/repos/poky-tx2$ cd foo/bar/
etienne@ubuntu:~/repos/poky-tx2/foo/bar$ ls
etienne@ubuntu:~/repos/poky-tx2/foo/bar$ source ../../oe-init-build-env
You had no conf/local.conf file. This configuration file has therefore been
created for you with some default values. You may wish to edit it to, for
example, select a different MACHINE (target hardware). See conf/local.conf
for more information as common configuration options are commented.
You had no conf/bblayers.conf file. This configuration file has therefore been
created for you with some default values. To add additional metadata layers
into your configuration please add entries to conf/bblayers.conf.
The Yocto Project has extensive documentation about OE including a reference
manual which can be found at:
http://yoctoproject.org/documentation
For more information about OpenEmbedded see their website:
http://www.openembedded.org/
### Shell environment set up for builds. ###
You can now run 'bitbake <target>'
Common targets are:
core-image-minimal
core-image-sato
meta-toolchain
meta-ide-support
You can also run generated qemu images with a command like 'runqemu qemux86'
etienne@ubuntu:~/repos/poky-tx2/foo/bar/build$ ls
conf