checkpatch: Add warnings for using strn(cat|cpy)

strn(cat|cpy) has a bad habit of not nul-terminating the destination,
resulting in constructions like

	strncpy(foo, bar, sizeof(foo) - 1);
	foo[sizeof(foo) - 1] = '\0';

However, it is very easy to forget about this behavior and accidentally
leave a string unterminated. This has shown up in some recent coverity
scans [1, 2] (including code recently touched by yours truly).

Fortunately, the guys at OpenBSD came up with strl(cat|cpy), which always
nul-terminate strings. These functions are already in U-Boot, so we should
encourage new code to use them instead of strn(cat|cpy).

[1] https://lists.denx.de/pipermail/u-boot/2021-March/442888.html
[2] https://lists.denx.de/pipermail/u-boot/2021-January/438073.html

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

scripts/checkpatch.pl

@@ -2365,6 +2365,12 @@ sub u_boot_line {
 		     "Use 'if (IS_ENABLED(CONFIG...))' instead of '#if or #ifdef' where possible\n" . $herecurr);
 	}
 
+	# prefer strl(cpy|cat) over strn(cpy|cat)
+	if ($line =~ /\bstrn(cpy|cat)\s*\(/) {
+		WARN("STRL",
+		     "strl$1 is preferred over strn$1 because it always produces a nul-terminated string\n" . $herecurr);
+	}
+
 	# use defconfig to manage CONFIG_CMD options
 	if ($line =~ /\+\s*#\s*(define|undef)\s+(CONFIG_CMD\w*)\b/) {
 		ERROR("DEFINE_CONFIG_CMD",