16ch PWM extension board

This board will receive control commands for multiple PWM channels over UART and then execute them.

The baud rate of UART is 921600.

The control packet is defined as follows:

struct servo_cmd_t {
    uint8_t header;
    uint16_t expected_period;
    uint16_t servo_cmd[16];
    uint16_t checksum;
} __attribute__((packed));