populate_sdk_ext.bbclass: add SDK_CONF_MANIFEST_EXCLUDE

populate_sdk_ext.bbclass: add SDK_CONF_MANIFEST_EXCLUDE

Add SDK_CONF_MANIFEST_EXCLUDE to enable excluding items in sdk-conf-manifest.

By default, files under conf/ are all added to sdk-conf-manifest, as the
manifest file is set to 'conf/*'.

However, there are situations where some configuration files under conf/
directory are not intended to be added to sdk-conf-manifest, thus adding
SDK_CONF_MANIFEST_EXCLUDE to enable users to do this.

This variable takes the form of glob matching.
e.g.
SDK_CONF_MANIFEST_EXCLUDE = "conf/autogen*"
This would exclude all files under conf/ starting with 'autogen' from
sdk-conf-manifest.

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