bootctl: move toupper() implementation to string-util.h

We already have tolower() calls there, hence let's unify this at one place.
Also, update the code to only use ASCII operations, so that we don't end up
being locale dependant.

src/boot/bootctl.c

@@ -597,15 +597,6 @@ error:
         return r;
 }
 
-static char* strupper(char *s) {
-        char *p;
-
-        for (p = s; *p; p++)
-                *p = toupper(*p);
-
-        return s;
-}
-
 static int mkdir_one(const char *prefix, const char *suffix) {
         char *p;
 
@@ -654,7 +645,7 @@ static int copy_one_file(const char *esp_path, const char *name, bool force) {
 
                 /* Create the EFI default boot loader name (specified for removable devices) */
                 v = strjoina(esp_path, "/EFI/BOOT/BOOT", name + strlen("systemd-boot"));
-                strupper(strrchr(v, '/') + 1);
+                ascii_strupper(strrchr(v, '/') + 1);
 
                 k = copy_file(p, v, force);
                 if (k < 0 && r == 0)