build: Support compile with GNU make 4.0

Signed-off-by: Brandon McAnsh <brandonm@matricom.net>
kernel: Use pre 4.4 paths for TARGET_KERNE_CUSTOM_TOOLCHAIN

Signed-off-by: Brandon McAnsh <brandonm@matricom.net>
dumpvar: Show off firmware version, toolchain, and ccache_base_dir

Signed-off-by: Brandon McAnsh <brandonm@matricom.net>
Allow ccache to be hit more effectively

* Also allows me to share the ccache between my trees
  and not suffer

Signed-off-by: Brandon McAnsh <brandonm@matricom.net>
Don't set CCACHE_BASEDIR if already defined

Signed-off-by: Brandon McAnsh <brandonm@matricom.net>