core-image.bbclass: Tweak the descriptions of the image features

core-image.bbclass: Tweak the descriptions of the image features

Some minor grammatical changes, and removal of some duplicate
information. Also note which features were previously part of the
debug-tweaks feature.

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>