This will be used to build our image as well as tools that need to go
into this image.

Notable changes from Buster:
* Python 2/pip2 module installations have been removed, since nothing
besides the main Python 2 binary (and virtualenv support) is now
available through Bullseye.
* In the cases where both the main library package and the development
package are being installed, now, only the dev package is specified. The
main library is typically marked as a dependency of the dev package.
This reduces the number of changes we have to make as SONAMEs change.

Signed-off-by: Saikrishna Arcot <sarcot@microsoft.com>

This adds the Makefile changes to use the Bullseye slave image, but
doesn't use it by default. There should be no functional changes with
this change (Buster will still be used for now).

Signed-off-by: Saikrishna Arcot <sarcot@microsoft.com>

Signed-off-by: Saikrishna Arcot <sarcot@microsoft.com>