CPU_CORTEX_M3_M4 replaces CPU_CORTEXM3 as the umbrella option for Cortex M3/M4
processors.
NOTE: Selecting CPU_CORTEXM4 still currently forces the selection of
CPU_CORTEXM3. Breaking that forced select will be done in a later commit.
Change-Id: I0f36b3a2adc5c6c66db4e9b6353b921199544deb
Signed-off-by: Peter Mitsis <peter.mitsis@windriver.com>