Fix GCC7 warnings about string truncation

GCC7 with -Wall warns about possible string truncation with
snprint()-type functions with "directive output may be truncated writing
1 byte into a region of size between 0 and 9
[-Werror=format-truncation=]" + "note: ‘snprintf’ output between 5 and
14 bytes into a destination of size 12" (or similar). Fix this by
increasing sizes of buffers.

See https://gcc.gnu.org/gcc-7/changes.html

Change-Id: Ib848f2a56dd658783534158947ae1be7c0e99d45
Signed-off-by: Freddie Chopin <freddie.chopin@gmail.com>
Reviewed-on: http://openocd.zylin.com/4175
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Tested-by: jenkins
Reviewed-by: Andreas Bolsch <hyphen0break@gmail.com>

src/flash/nor/kinetis.c

@@ -1959,7 +1959,7 @@ static int kinetis_probe_chip(struct kinetis_chip *k_chip)
 	unsigned cpu_mhz = 120;
 	unsigned idx;
 	bool use_nvm_marking = false;
-	char flash_marking[8], nvm_marking[2];
+	char flash_marking[11], nvm_marking[2];
 	char name[40];
 
 	k_chip->probed = false;