unified: move basic ticks-to-ms conversion to kernel.h

The basic conversion, i.e. not handling the TICKS_UNLIMITED case, is
useful internally since the kernel is still tick-based.

Change-Id: I00a01047ec48dad6834dd8ea5dc831eb8c0c2501
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>

include/kernel.h

@@ -237,6 +237,16 @@ extern void *k_thread_custom_data_get(void);
  *  kernel timing
  */
 
+#include <sys_clock.h>
+
+/* private internal time manipulation (users should never play with ticks) */
+
+static int64_t __ticks_to_ms(int64_t ticks)
+{
+	return (MSEC_PER_SEC * (uint64_t)ticks) / sys_clock_ticks_per_sec;
+}
+
+
 /* timeouts */
 
 struct _timeout;