STM32F1: Support M997 - Reset and begin flash (#15209)

Marlin/src/HAL/HAL_STM32F1/HAL.cpp

@@ -369,4 +369,6 @@ void analogWrite(pin_t pin, int pwm_val8) {
     analogWrite(uint8_t(pin), pwm_val8);
 }
 
+void flashFirmware(int16_t value) { nvic_sys_reset(); }
+
 #endif // __STM32F1__

Marlin/src/HAL/HAL_STM32F1/HAL.h

@@ -256,3 +256,6 @@ void analogWrite(pin_t pin, int pwm_val8); // PWM only! mul by 257 in maple!?
 
 #define JTAG_DISABLE() afio_cfg_debug_ports(AFIO_DEBUG_SW_ONLY)
 #define JTAGSWD_DISABLE() afio_cfg_debug_ports(AFIO_DEBUG_NONE)
+
+#define PLATFORM_M997_SUPPORT
+void flashFirmware(int16_t value);