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

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

Change-Id: I741c0ee11fc44703b31c5a97220f83cbb127b82c
Signed-off-by: Lokesh Chamane <lokesh.c703@gmail.com>