Allow the user to provide additional packages to this image.
This lets core-image-basic behave like all other core-image*
recipes (which do support CORE_IMAGE_EXTRA_INSTALL), as well
as match the documentation which suggests this as the mode to
extend any core-image* image.
v2 - drop redundant setting of CORE_IMAGE_EXTRA_INSTALL
Signed-off-by: Gary Thomas <gary@mlbassoc.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
