Allow defining multiple GCC O Levels [1/2]

Available GCC Optimization Levels
    -Os
    -O2 (Default)
    -O3

PS2: Derp fix
PS3: Derp fix 2.0

Change-Id: I55fa04026df24841b9d67325242ce3cb460726a1
Signed-off-by: Lokesh Chamane <lokesh.c703@gmail.com>

Conflicts:
	core/combo/TARGET_linux-arm.mk
	core/combo/select.mk