* Add a generic edit_metadata_file() function to modify variable
assignments in any metadata file (conf, bb, bbappend) using a callback
for flexibility
* Add a specific edit_bblayers_conf() function to modify
conf/bblayers.conf and add and/or remove layers from the BBLAYERS
value within it.
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>