Makefile: adding config variable for Os/O2/O3 compilation

Signed-off-by: Dan Trifan <jimsth@gmail.com>
defconfig: optimize -O3

Signed-off-by: Dan Trifan <jimsth@gmail.com>
fix compilation with -03 and gcc4.8

* credits: showp1984 and LaboDJ

Signed-off-by: Dan Trifan <jimsth@gmail.com>