stm32/boards/PYBD_SF2: Put mbedtls library code in external QSPI flash.

mbedtls is large and self contained so is a good candidate to be in
external XIP flash, to keep enough spare ROM in internal flash.

ports/stm32/boards/PYBD_SF2/f722_qspi.ld

@@ -47,6 +47,8 @@ SECTIONS
 {
     .text_ext :
     {
+        . = ALIGN(4);
+        *lib/mbedtls/*(.text* .rodata*)
         . = ALIGN(512);
         *(.big_const*)
         . = ALIGN(4);