Makefile: use -O3 where possible, if not able use -O2 instead
Signed-off-by: Paul Beeler <sparksco@gmail.com>
defconfig:  use XZ kernel compression
Signed-off-by: Paul Beeler <sparksco@gmail.com>
defconfig:  use -O3 instead of -Os
Signed-off-by: Paul Beeler <sparksco@gmail.com>