Disable interrupts during EEFC_ReadUniqueID()

Reading the Unique ID from flash is a rather tricky procedure: After the
STUI command has been issued, we cannot read normal flash anymore.
Rather, the unique ID is mapped at 0x00000000. This is unfortuantely
also where the exception vector table is stored.

EEFC_ReadUniqueID() is already linked to RAM, which is good.  Hoewver,
if an Interrupt happens between STUI and SPUI, then we try to access
the vector table and code from flash, which is illegal.  We run into
a hardfault and stay there until the watchdog resets the processor.

Change-Id: I3c4fad55b47e9013f6615a331983b3989ca805a7
Closes: OS#4428

firmware/atmel_softpack_libraries/libchip_sam3s/source/unique_id.c

@@ -8,6 +8,11 @@ void EEFC_ReadUniqueID(unsigned int *pdwUniqueID)
 {
 	unsigned int status;
 
+	/* disable interrupts, as interrupt vectors are stored in flash,
+	 * and after STUI was issued, we can no longer access flassh until
+	 * SPUI complets */
+	__disable_irq();
+
 	/* Errata / Workaround: Set bit 16 of EEFC Flash Mode Register
 	 * to 1 */
 	EFC->EEFC_FMR |= (1 << 16);
@@ -40,4 +45,6 @@ void EEFC_ReadUniqueID(unsigned int *pdwUniqueID)
 	do {
 		status = EFC->EEFC_FSR;
 	} while ((status & EEFC_FSR_FRDY) != EEFC_FSR_FRDY);
+
+	__enable_irq();
 }