Fix LFCLK sources

LFCLK source was hard-coded to be RC oscillator, now supports all other
sources from the tools menu

Author: sandeepmistry

cores/nRF5/wiring.c

@@ -31,7 +31,13 @@ void init( void )
   NVIC_ClearPendingIRQ(RTC1_IRQn);
   NVIC_EnableIRQ(RTC1_IRQn);
 
-  NRF_CLOCK->LFCLKSRC = (uint32_t)((CLOCK_LFCLKSRC_SRC_RC << CLOCK_LFCLKSRC_SRC_Pos) & CLOCK_LFCLKSRC_SRC_Msk);
+  #if defined(USE_LFXO)
+    NRF_CLOCK->LFCLKSRC = (uint32_t)((CLOCK_LFCLKSRC_SRC_Xtal << CLOCK_LFCLKSRC_SRC_Pos) & CLOCK_LFCLKSRC_SRC_Msk);
+  #elif defined(USE_LFSYNT)
+    NRF_CLOCK->LFCLKSRC = (uint32_t)((CLOCK_LFCLKSRC_SRC_Synth << CLOCK_LFCLKSRC_SRC_Pos) & CLOCK_LFCLKSRC_SRC_Msk);
+  #else //USE_LFRC
+    NRF_CLOCK->LFCLKSRC = (uint32_t)((CLOCK_LFCLKSRC_SRC_RC << CLOCK_LFCLKSRC_SRC_Pos) & CLOCK_LFCLKSRC_SRC_Msk);
+  #endif
   NRF_CLOCK->TASKS_LFCLKSTART = 1UL;
 
   NRF_RTC1->PRESCALER = 0;