arm: stm32f4: Do not enable USART1/USART2 by default

Not all the boards (for instance the Nuclo F412) use USART1 or USART2.
Let each board enable these USARTs when really used.

Change-Id: Idfe79c724bd7b1ab154310b4a8234b52eef2298d
Signed-off-by: Florian Vaussard <florian.vaussard@heig-vd.ch>
1 file changed