CI: restrict compression threading

On large systems, using all of the CPUs and 50% of the RAM when xz
compressing packages is actively harmful because it will happily use up
to that limit.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

ci/base.yml

@@ -30,6 +30,9 @@ local_conf_header:
     CONF_VERSION = "2"
     BB_NUMBER_THREADS = "16"
     PARALLEL_MAKE = "-j16"
+    XZ_MEMLIMIT = "25%"
+    XZ_THREADS = "16"
+    ZSTD_THREADS = "16"
     LICENSE_FLAGS_ACCEPTED += "Arm-FVP-EULA"
   setup: |
     PACKAGE_CLASSES = "package_ipk"