packagegroup-connectivity: Move dropbear-related packages to oniro-image-common.inc

Upstream[1] drops the complementary package openssh based on
PACKAGE_INSTALL (which uses IMAGE_INSTALL). This means that by default,
the drop won't be effective when we pull the dependency explicitly
through RDEPENDS on a packagegroup.

To take advantage of the above-mentioned upstream workaround, we start
pulling the SSH provider (dropbear in our case) directly from the images
(with IMAGE_INSTALL) using the dropbear existing packagegroup. This
package group will pull in both the dropbear and an SFTP server package
as a package recommendation.

[1] https://bugzilla.yoctoproject.org/show_bug.cgi?id=14858



Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>