cmd: test: add bug-compatibility special case for 'test -n'

It turns out that there is lots of code in the wild, including in the
U-Boot tree itself, which used to rely on

  test -n $somevar

to yield false when $somevar is not defined or empty. See for example
all the occurrences of 'test -n $fdtfile'. That was really only a
quirk of the implementation that refused calls with argc < 3, and not
because it was interpreted as

  test -n "$somevar"

which is how this should be spelled.

While not exactly conforming to POSIX, we can accomodate such scripts
by special-casing a single argument "-n" to be interpreted as if it
comes from code as above with empty $somevar.

Since we only just added the ability to test a string for emptiness
using the single-argument form, it is very unlikely that there is code
doing

  test "$str"

which would now fail if $str happens to be exactly "-n"; such a test
should really always be spelled

  test -n "$str"

Fixes: 8b0619579b ("cmd: test: fix handling of single-argument form of test")
Reported-by: Franz Schnyder <franz.schnyder@toradex.com>
Signed-off-by: Rasmus Villemoes <ravi@prevas.dk>
Reviewed-by: Simon Glass <sjg@chromium.org>

cmd/test.c

@@ -75,12 +75,25 @@ static int do_test(struct cmd_tbl *cmdtp, int flag, int argc,
 	 * Per POSIX, 'test' with 0 arguments should return 1, while
 	 * 'test <arg>' should be equivalent to 'test -n <arg>',
 	 * i.e. true if and only if <arg> is not empty.
+	 *
+	 * However, due to previous versions of U-Boot unconditionally
+	 * returning false when 'test' was given less than two
+	 * arguments, there are existing scripts that do
+	 *
+	 *   test -n $somevar
+	 *
+	 * (i.e. without properly quoting $somevar) and expecting that
+	 * to return false when $somevar expands to nothing. It is
+	 * quite unlikely that anyone would use the single-argument
+	 * form to test a string for being empty and a possible
+	 * non-empty value for that string to be exactly "-n". So we
+	 * interpret 'test -n' as if it was 'test -n ""'.
 	 */
 	if (argc < 2)
 		return 1;
 
 	if (argc == 2)
-		return !strcmp(argv[1], "");
+		return !strcmp(argv[1], "") || !strcmp(argv[1], "-n");
 
 #ifdef DEBUG
 	{