bitbake-layers: fix error handling in add-layer / remove-layer

* Fix add-layer error message when a layer is already in BBLAYERS
* Ensure we show an error message if we can't find BBLAYERS at all

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
utils: make edit_metadata_file() handle VARIABLE +=

This is a little crude as the usage basically ignores that you're doing
an append operation, but for a lot of cases it will be sufficient.

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