Why does the yocto bblayers.conf file use absolute paths?

You can use relative paths in bblayers.conf.

There is probably this line in your bblayers.conf:

BBPATH = "${TOPDIR}"

When you want to find out this variable's content, you will probably find the top-level directory of your build directory:

bitbake -e | grep ^TOPDIR
# searches for bitbake variables

Inside this directory you could create a layer meta-test and add it in bblayers.conf with a relative path:

BBLAYERS ?= " \
  meta-test \
  [...]
  "

So the answer on your question why there are absolute paths in bblayers.conf is that you can place your build directory anywhere on the system and not dependant from Yocto.

Relative Paths to Layers must always be relative to the build directory.

I have managed to get "relative paths" in bblayers.conf files working by replacing

BBLAYERS ?= " \
  /home/username/poky/meta \
  ...

with

BBLAYERS ?= " \
  ${TOPDIR}/../meta \
  ...

I guess one caveat with this approach is that I'm relying on the meta-XXX layer directories always being in the parent folder of TOPDIR. This seems to be the case with the default way of using yocto, but might not be so for more customized build setups.

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

I found a way to use relative paths.

You can use inline python to traverse the file system. The following script uses the provided TOPDIR variable and then navigates to its parent via python's os.path api.

# LAYER_CONF_VERSION is increased each time build/conf/bblayers.conf
# changes incompatibly
LCONF_VERSION = "6"

BBPATH = "${TOPDIR}"
BBFILES ?= ""

YOCTOROOT = "${@os.path.abspath(os.path.join("${TOPDIR}", os.pardir))}"

BBLAYERS ?= " \
  ${YOCTOROOT}/poky/meta \
  ${YOCTOROOT}/poky/meta-yocto \
  ${YOCTOROOT}/poky/meta-yocto-bsp \
"

BBLAYERS_NON_REMOVABLE ?= " \
  ${YOCTOROOT}/poky/meta \
  ${YOCTOROOT}/poky/meta-yocto \
"

References

• OpenEmbedded's bitbake.conf
• Advanced functionality with python
• How do I get the parent directory in Python?