Add support for SAM3/4 basic TERMIOS and flow control.  There are issues with IFLOW control: PDC or DMAC support is required

STM32 Serial:  Don't compile up_earlyserialinit() if USE_EARLYSERIALINIT is not define.  Otherwise, a macro definition clobbers the function definition

Move selection for CONFIG_SERIAL_TERMIOS out of MCU Kconfigs to common drivers/serial/Kconfig.  Add CONFIG_ARCH_HAVE_SERIAL_TERMIOS to indicate if an MCU supports TERMIOS

EFM32 SPI: setfrequency() needs to return actual frequency; simplify nbits usages by removing conditional logic