[readline] Add init_editstring() wrapper function

Standardise on using init_editstring() to initialise an embedded
editable string, to match the coding style used by other embedded
objects.

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

src/hci/mucurses/widgets/editbox.c

 		    WINDOW *win, unsigned int row, unsigned int col,
 		    unsigned int width, unsigned int flags ) {
 	memset ( box, 0, sizeof ( *box ) );
-	box->string.buf = buf;
-	box->string.len = len;
+	init_editstring ( &box->string, buf, len );
 	box->string.cursor = strlen ( buf );
 	box->win = ( win ? win : stdscr );
 	box->row = row;

src/hci/readline.c

 		printf ( "%s", prompt );
 
 	memset ( &string, 0, sizeof ( string ) );
-	string.buf = buf;
-	string.len = sizeof ( buf );
+	init_editstring ( &string, buf, sizeof ( buf ) );
 	buf[0] = '\0';
 
 	while ( 1 ) {

src/include/ipxe/editstring.h

 	unsigned int mod_end;
 };
 
+/**
+ * Initialise editable string
+ *
+ * @v string		Editable string
+ * @v buf		Buffer for string
+ * @v len		Length of buffer
+ */
+static inline void init_editstring ( struct edit_string *string, char *buf,
+				     size_t len ) {
+	string->buf = buf;
+	string->len = len;
+}
+
 extern int edit_string ( struct edit_string *string, int key ) __nonnull;
 
 #endif /* _IPXE_EDITSTRING_H */