copy_buildsystem: add ability to exclude layers

copy_buildsystem: add ability to exclude layers

In some cases, we may have some kind of download layers in BBLAYERS, so
that we can set BB_NO_NETWORK to "1". This results in extremely large
extensible SDK. And we actually don't need these download layers in the
SDK.

Add a new variable, SDK_LAYERS_EXCLUDE, to enable users to explicitly
exclude some layers when generating the extensible SDK.

[YOCTO #8878]

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>