Install usrmerge on ubuntu (needed for RTEMS builds)

- travis: only do that for RTEMS
  (as precise & trusty don't have usrmerge)
- gitlab: use focal instead
  (apt install usrmerge doesn't work in Docker)

travis/.travis.yml.example-full

@@ -104,9 +104,9 @@ jobs:
 
 # Cross-compilation to RTEMS
 
-  - env: BASE=7.0 RTEMS=4.10
+  - env: BASE=7.0 RTEMS=4.10 APT=usrmerge
 
-  - env: BASE=7.0 RTEMS=4.9
+  - env: BASE=7.0 RTEMS=4.9 APT=usrmerge
 
 # Other gcc versions (added as an extra package)