Since kernel v3.14, we support protocol v3 in the kernel. (chrome-os-partner:20257)
Therefore, for newer board or newer firmware revision where we are sure the AP is using v3.14 or later, we can remove v2 support.

stm32 i2c already only support v3.
stm32 spi still support v2 if CONFIG_SPI_PROTOCOL_V2 is compiled in.
xxx lpc still support v2,

Still, even when only v3 is supported, we are rather inefficient: We translate the "v3 message" (packet) into a "v2 message" (command) and calling the older style command processing function: host_command_received()

We should be able to squeeze some flash and cycle there.