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/basic/string-util.h

@@ -137,6 +137,9 @@ char ascii_tolower(char x);
 char *ascii_strlower(char *s);
 char *ascii_strlower_n(char *s, size_t n);
 
+char ascii_toupper(char x);
+char *ascii_strupper(char *s);
+
 int ascii_strcasecmp_n(const char *a, const char *b, size_t n);
 int ascii_strcasecmp_nn(const char *a, size_t n, const char *b, size_t m);