build: consolidate multiple variables into universal-obj-y

build: consolidate multiple variables into universal-obj-y

The directory descent mechanism, and a less-flat tree both helped
in making some *-obj-y definitions very short.  Many of these
often end up in universal-obj-y, and used to be separate only
because of libuser (which is now part of history...).

Consolidate these variables in a single one.

Signed-off-by: Paolo Bonzini <[email protected]>