libc: minimal: Fix handling of %f conversion specifiers for inf & nan

The C standard says that %f should use '[-]inf' or '[-]infinity' (which
style is implementation defined) for infinity handling and '[-]nan' for
NaN.

We where adding a '+' and had the wrong case for 'inf' and 'nan'.

Before -> After

+INF -> inf
-INF -> -inf
 NaN -> nan

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>

lib/libc/minimal/source/stdout/prf.c

@@ -226,14 +226,16 @@ static int _to_float(char *buf, uint64_t double_temp, int c,
 
 	if (exp == 0x7ff) {
 		if (!fract) {
-			*buf++ = sign ? '-' : '+';
-			*buf++ = 'I';
-			*buf++ = 'N';
-			*buf++ = 'F';
+			if (sign) {
+				*buf++ = '-';
+			}
+			*buf++ = 'i';
+			*buf++ = 'n';
+			*buf++ = 'f';
 		} else {
-			*buf++ = 'N';
+			*buf++ = 'n';
 			*buf++ = 'a';
-			*buf++ = 'N';
+			*buf++ = 'n';
 		}
 		*buf = 0;
 		return buf - start;

tests/lib/sprintf/src/main.c

@@ -74,20 +74,20 @@ void test_sprintf_double(void)
 	var.u1 = 0x00000000;
 	var.u2 = 0x7ff00000;    /* Bit pattern for +INF (double) */
 	sprintf(buffer, "%f", var.d);
-	zassert_true((strcmp(buffer, "+INF") == 0),
-		     "sprintf(+INF) - incorrect output '%s'\n", buffer);
+	zassert_true((strcmp(buffer, "inf") == 0),
+		     "sprintf(inf) - incorrect output '%s'\n", buffer);
 
 	var.u1 = 0x00000000;
 	var.u2 = 0xfff00000;    /* Bit pattern for -INF (double) */
 	sprintf(buffer, "%f", var.d);
-	zassert_true((strcmp(buffer, "-INF") == 0),
+	zassert_true((strcmp(buffer, "-inf") == 0),
 		     "sprintf(-INF) - incorrect output '%s'\n", buffer);
 
 	var.u1 = 0x00000000;
 	var.u2 = 0x7ff80000;    /* Bit pattern for NaN (double) */
 	sprintf(buffer, "%f", var.d);
-	zassert_true((strcmp(buffer, "NaN") == 0),
-		     "sprintf(NaN) - incorrect output '%s'\n", buffer);
+	zassert_true((strcmp(buffer, "nan") == 0),
+		     "sprintf(nan) - incorrect output '%s'\n", buffer);
 
 	var.d = 1.0;
 	sprintf(buffer, "%f", var.d);