lib/bb/utils: improve edit_bblayers_conf() handling of bblayers.conf formatting

lib/bb/utils: improve edit_bblayers_conf() handling of bblayers.conf formatting

Make the following improvements to edit_bblayers_conf():

* Support ~ in BBLAYERS entries
* Handle where BBLAYERS items are added over multiple lines with +=
  instead of one single long item

Also add some comments documenting the function arguments and return
values as well as a set of bitbake-selftest tests.

(This function is used by the bitbake-layers add, remove and
layerindex-fetch subcommands, as well as devtool when adding the
workspace layer).

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>