fix: 'reset dfu' on the tinySA ULTRA

- 'reset dfu' and enter_dfu() are now also built for the tinySA4 (the
  DFU menu stays tinySA3 only); the reply is sent before the reset
- F303 __early_init(): the jump to the system bootloader failed and the
  bootloader reset the MCU. crt0 calls __early_init() on the process
  stack with interrupts disabled, so switch to the main stack
  (CONTROL = 0) and enable interrupts before the jump, as in the F072
  board.c. Also enable the SYSCFG clock for the system memory remap and
  replace the logical || that made it write 1 to SYSCFG_CFGR1

Tested on HW V0.4.5.1: 'reset dfu' enters the STM32 BOOTLOADER and
dfu-util can flash it.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

NANOVNA_STM32_F303/board.c

@@ -114,11 +114,15 @@ void __early_init(void) {
     bootloader();
     while(1);
    #else
-    __disable_irq();
-    uint32_t foo = SYSCFG->CFGR1;
-    foo = (foo & ~SYSCFG_CFGR1_MEM_MODE) || SYSCFG_CFGR1_MEM_MODE_0;
-    SYSCFG->CFGR1 = foo;
+    // Map the system memory at 0 so interrupts use the bootloader vectors (SYSCFG needs its clock)
+    RCC->APB2ENR |= RCC_APB2ENR_SYSCFGEN;
+    SYSCFG->CFGR1 = (SYSCFG->CFGR1 & ~SYSCFG_CFGR1_MEM_MODE) | SYSCFG_CFGR1_MEM_MODE_0;
     __DSB();
+    __ISB();
+    // The bootloader needs interrupts, crt0 disabled them (as in the F072 board.c)
+    __enable_irq();
+    // and the main stack, crt0 switched to the process stack
+    __set_CONTROL(0);
     __ISB();
 
       __set_MSP(*((uint32_t *)(STM32F303xC_SYSTEM_MEMORY)));

main.c

@@ -578,15 +578,14 @@ VNA_SHELL_FUNCTION(cmd_reset)
 {
   (void)argc;
   (void)argv;
-#ifndef TINYSA4
   if (argc == 1) {
     if (get_str_index(argv[0], "dfu") == 0) {
       shell_printf("Performing reset to DFU mode\r\n");
+      chThdSleepMilliseconds(100);        // let USB send the text
       enter_dfu();
       return;
     }
   }
-#endif
   shell_printf("Performing reset\r\n");
   rccEnableWWDG(FALSE);
   WWDG->CFR = 0x60;

ui.c

@@ -750,22 +750,21 @@ extern const char *states[];
   lcd_set_font(FONT_SMALL);
 }
 
-#ifdef __HAS_DFU__
+// Also used by "reset dfu" on the tinySA4, which has no DFU menu (__HAS_DFU__)
 void
 enter_dfu(void)
 {
   int x = 5, y = 5;
   ili9341_set_foreground(LCD_FG_COLOR);
   ili9341_set_background(LCD_BG_COLOR);
-  // leave a last message 
+  // leave a last message
   ili9341_clear_screen();
   ili9341_drawstring_7x13("DFU: Device Firmware Update Mode\n"
                           "To exit DFU mode, please reset device yourself.", x, y);
-  // see __early_init in ./NANOVNA_STM32_F072/board.c
+  // see __early_init in ./NANOVNA_STM32_F072/board.c and ./NANOVNA_STM32_F303/board.c
   *((unsigned long *)BOOT_FROM_SYTEM_MEMORY_MAGIC_ADDRESS) = BOOT_FROM_SYTEM_MEMORY_MAGIC;
   NVIC_SystemReset();
 }
-#endif
 
 static void
 select_lever_mode(int mode)