Add support for boards with 16MHz HSE crystal. Note to use this feature users must add -D XTAL16M to the appropriate build options in boards.txt, as there are too few boards with 16MHz crystals to warrant adding this as a separate menu to each board in boards.txt

STM32F1/cores/maple/libmaple/rcc_f1.c

@@ -134,6 +134,12 @@ void rcc_configure_pll(rcc_pll_cfg *pll_cfg) {
     cfgr &= ~(RCC_CFGR_PLLSRC | RCC_CFGR_PLLMUL);
     cfgr |= pll_cfg->pllsrc | pll_mul;
 
+#ifdef XTAL16M
+    // 16MHz crystal (HSE)
+    // in this case we additionally set the Bit 17 (PLLXTPRE=1)  =>  then HSE clock is divided by 2 before PLL entry
+    cfgr |= RCC_CFGR_PLLXTPRE;
+#endif	
+	
     RCC_BASE->CFGR = cfgr;
 }