lib/oe/copy_buildsystem: fix building eSDK with indirect paths in BBLAYERS

lib/oe/copy_buildsystem: fix building eSDK with indirect paths in BBLAYERS

Indirect paths (e.g. ${TOPDIR}/../meta-something) do generally work if
used in BBLAYERS in bblayers.conf. However, if you built an extensible
SDK with this configuration then the creation of the workspace within
the SDK using devtool in do_populate_sdk_ext failed. This is because
the copy_buildsystem code was no longer correctly recognising that the
core layer ("meta") was part of a repository (e.g. openembedded-core /
poky) that should be shipped together - because of the indirection - and
thus it was splitting out the meta directory, and a number of places in
the code assume that the meta directory is next to the scripts
directory. Use os.path.abspath() to flatten out any indirections.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>