Add KEEP to init_array in common .ld file

To ensure global constructors are call. See:
http://stackoverflow.com/questions/6343348/global-constructor-call-not-i
n-init-array-section

linker_scripts/gcc/nrf5x_common.ld

@@ -107,8 +107,8 @@ SECTIONS
 		. = ALIGN(4);
 		/* init data */
 		PROVIDE_HIDDEN (__init_array_start = .);
-		*(SORT(.init_array.*))
-		*(.init_array)
+		KEEP(*(SORT(.init_array.*)))
+		KEEP(*(.init_array))
 		PROVIDE_HIDDEN (__init_array_end = .);