[prompt] Replace shell_banner() with a generic prompt() function

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/usr/prompt.h

@@ -0,0 +1,14 @@
+#ifndef _USR_PROMPT_H
+#define _USR_PROMPT_H
+
+/** @file
+ *
+ * Prompt for keypress
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER );
+
+extern int prompt ( const char *text, unsigned int wait_ms, int key );
+
+#endif /* _USR_PROMPT_H */