Rework if statements for GCC Level. [1/2]

Reworked the if statements to give only 2 options, either Os or O3. If neither one of them is selected as an option, it will select the default Optimization Level which is O2.

PS2: Update commit message
PS3: Rebase and ready for merge

Change-Id: I64d5787bc86242fd613eb3931cd9bd46d7f17c49
Signed-off-by: Lokesh Chamane <lokesh.c703@gmail.com>

Conflicts:

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