string.h: Add isprint()

Change-Id: If179687f0a15cf5b16723ad18d8eb86a2d5fa48d
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/c/29479
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/include/string.h

@@ -156,6 +156,11 @@ static inline int isspace(int c)
 	}
 }
 
+static inline int isprint(int c)
+{
+	return c >= ' ' && c <= '~';
+}
+
 static inline int isdigit(int c)
 {
 	return (c >= '0' && c <= '9');