bblayers.conf.sample: Fix empty BBPATH entry warnings

Many people are seeing issues from the empty path warnings from BBPATH.
The empty path entry corresponding to the current working directory is a
problem since if cwd changes, so does BBPATH and build reproducibility.

Simply removing the empty element causes problems since the build
directory then isn't listed in BBPATH which means local.conf isn't found
and this gives an extremely confusing error message about bbappends
being unsatisfied.

The build directory in bitbake terms is TOPDIR. The correct way to fix
things is to add in TOPDIR into bblayers.conf itself. This means the
layers can happily append/prepend to BBPATH at will as its no longer
empty hence neatly solving all the various problems. Since the file has
changed, the version is also changed.

(From meta-yocto rev: 7bbbedf64b0820cacffe723789486d3081894827)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta-yocto/conf/bblayers.conf.sample

@@ -1,8 +1,10 @@
 # LAYER_CONF_VERSION is increased each time build/conf/bblayers.conf
 # changes incompatibly
-LCONF_VERSION = "4"
+LCONF_VERSION = "5"
 
+BBPATH = "${TOPDIR}"
 BBFILES ?= ""
+
 BBLAYERS ?= " \
   ##COREBASE##/meta \
   ##COREBASE##/meta-yocto \