stm32/boards/stm32h743.ld: Fix total flash size, should be 2048k.

Fixes issue #4240.

ports/stm32/boards/stm32h743.ld

@@ -5,7 +5,7 @@
 /* Specify the memory areas */
 MEMORY
 {
-    FLASH (rx)      : ORIGIN = 0x08000000, LENGTH = 1024K
+    FLASH (rx)      : ORIGIN = 0x08000000, LENGTH = 2048K
     FLASH_ISR (rx)  : ORIGIN = 0x08000000, LENGTH = 128K    /* sector 0, 128K */
     FLASH_FS (r)    : ORIGIN = 0x08020000, LENGTH = 128K    /* sector 1, 128K */
     FLASH_TEXT (rx) : ORIGIN = 0x08040000, LENGTH = 1792K   /* sectors 6*128 + 8*128 */