Portenta H7: Don't blink BLUE LED

This was caused by the default M4 core sketch when no other M4 sketch is configured/loaded.

Disable the automatic starting of the M5 core.

Was leaving the BLUE led on, so initVariant turns it off...

Author: KurtE

variants/arduino_portenta_h7_stm32h747xx_m7/arduino_portenta_h7_stm32h747xx_m7.conf

@@ -95,3 +95,4 @@ CONFIG_VIDEO_BUFFER_POOL_ALIGN=32
 CONFIG_VIDEO_BUFFER_USE_SHARED_MULTI_HEAP=y
 CONFIG_VIDEO_BUFFER_SMH_ATTRIBUTE=2
 CONFIG_FLASH=y
+CONFIG_STM32H7_BOOT_M4_AT_INIT=n 

variants/arduino_portenta_h7_stm32h747xx_m7/variant.cpp

@@ -6,3 +6,14 @@ void _on_1200_bps() {
     *(__IO uint32_t *)tmp = (uint32_t)0xDF59;
     NVIC_SystemReset();
 }
+
+void initVariant(void) {
+    // check the BLUE LED
+    /* Set led1 inactive since the Arduino bootloader leaves it active */
+    const struct gpio_dt_spec led2 = GPIO_DT_SPEC_GET(DT_ALIAS(led2), gpios);
+    if (!gpio_is_ready_dt(&led2)) {
+        return;
+    }
+
+    gpio_pin_configure_dt(&led2, GPIO_OUTPUT_INACTIVE);
+}