libbb: introduce and use strftime_[YYYYMMDD]HHMMSS()

function                                             old     new   delta
strftime_fmt                                           -      53     +53
strftime_YYYYMMDDHHMMSS                                -      12     +12
strftime_HHMMSS                                        -      12     +12
human_time                                            44      43      -1
fmtstr_t                                               9       -      -9
step_time                                            361     345     -16
watch_main                                           261     232     -29
------------------------------------------------------------------------------
(add/remove: 3/1 grow/shrink: 0/3 up/down: 77/-55)             Total: 22 bytes
   text	   data	    bss	    dec	    hex	filename
 919203	    932	  17692	 937827	  e4f63	busybox_old
 919209	    932	  17692	 937833	  e4f69	busybox_unstripped

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/coreutils/stat.c b/coreutils/stat.c
index c8677eb..dc9d81c 100644
--- a/coreutils/stat.c
+++ b/coreutils/stat.c
@@ -127,7 +127,7 @@
 	/*static char buf[sizeof("YYYY-MM-DD HH:MM:SS.000000000")] ALIGN1;*/
 #define buf bb_common_bufsiz1
 
-	strftime(buf, sizeof(buf), "%Y-%m-%d %H:%M:%S.000000000", localtime(&t));
+	strcpy(strftime_YYYYMMDDHHMMSS(buf, sizeof(buf), &t), ".000000000");
 	return buf;
 #undef buf
 }