blackpill-f4: Unmap BootROM after ST DFU done

* After using ST MaskROM for initial reprogramming of BMD bootloader (or BMF),
  code gets to run with `SYSCFG_MEMRMP=0b01` System memory still mapped on 0x0,
  and with a `SCB_VTOR` of 0 any interrupts will invoke MaskROM handlers, crashing firmware.
* Unmap it both in BMD bootloader and in BMF, restoring normal operation
  and eliminating the need for initial manual/double reset (by pushbuttons).

src/platforms/common/blackpill-f4/blackpill-f4.c

@@ -72,6 +72,12 @@ void platform_init(void)
  scb_reset_core();
  }
 #endif
+
+ /* Unmap ST MaskROM and map back Internal Flash */
+ rcc_periph_clock_enable(RCC_SYSCFG);
+ if ((SYSCFG_MEMRM & 3U) == 1U)
+ SYSCFG_MEMRM &= ~3U;
+
  rcc_clock_setup_pll(&rcc_hse_25mhz_3v3[PLATFORM_CLOCK_FREQ]);
 
  /* Set up DM/DP pins. PA9/PA10 are not routed to USB-C. */

src/platforms/common/blackpill-f4/usbdfu.c

@@ -23,6 +23,7 @@
 #include <libopencm3/cm3/scb.h>
 #include <libopencm3/stm32/rcc.h>
 #include <libopencm3/stm32/gpio.h>
+#include <libopencm3/stm32/syscfg.h>
 #include <libopencm3/usb/dwc/otg_fs.h>
 
 #include "usbdfu.h"
@@ -54,6 +55,11 @@ int main(void)
  } else
  dfu_jump_app_if_valid();
 
+ /* Unmap ST MaskROM and map back Internal Flash */
+ rcc_periph_clock_enable(RCC_SYSCFG);
+ if ((SYSCFG_MEMRM & 3U) == 1U)
+ SYSCFG_MEMRM &= ~3U;
+
  rcc_clock_setup_pll(&rcc_hse_25mhz_3v3[PLATFORM_CLOCK_FREQ]);
 
  /* Assert blue LED as indicator we are in the bootloader */