Target changes in OpenJDK

[Gábor Boskovits]

Hello guix,

Starting from OpenJDK 11 the jre target is deprecated. It can still be
built using a separate make target.

I see three options here:
1. Remove the separate jdk output and put the jdk to the out otuput.
This will be the way upstream will go anyway.
2. Build the jre using the provided target: legacy-jre-image, and
leave outputs as is.
3. Do as 1, but build the jre, and add a jre output.

WDYT?