cmd: mbr: return false instead of NULL from bool found_key()

found_key() is declared to return bool but returns NULL when strdup()
fails. NULL is a pointer constant; while it happens to convert to
zero (i.e. false) it is a type mismatch that trips stricter
compilers/static analysers.

Return false to match the declared return type.

Signed-off-by: Naveen Kumar Chaudhary <naveen.osdev@gmail.com>

cmd/mbr.c

@@ -73,7 +73,7 @@ static bool found_key(const char *str, const char *key)
 
 	strcopy = strdup(str);
 	if (!strcopy)
-		return NULL;
+		return false;
 
 	s = strcopy;
 	while (s) {