makefile: add optimisation options

-o3, -o2 or -os

ffast math, lto, graphite

Change-Id: I7ece685e9403abb41f0ae4b42718759566628111
defconfig: enable optimisation options

-o3, ffast math, lto, graphite

Change-Id: Ibe033627d4756db8222ed5c979d959e1ec187cdb