imagebuilder: fix local packages/ folder

imagebuilder: fix local packages/ folder

This commit fixes commit "2999f810ff: build,IB: include kmods only in
local builds" which cause the local packages/ folder only to be added
for local builds but no longer for ImageBuilder created by the Buildbot.

The commits intention was to use remote kmods repositories rather than
storing them locally. Accidentally the entire handling of the local
`packages/` was removed.

Re-add the folder and include a README describing what it can be used
for.

Signed-off-by: Paul Spooren <mail@aparcar.org>
(cherry picked from commit 15e55a2190ba087679b24b8844a51a6e4d512cf3)
Fixes: #5068
Signed-off-by: Jo-Philipp Wich <jo@mein.io>