menu: add support for client defined statusline function

Currently displaying status line is done in a weak function
menu_display_statusline().

bootmenu.c overrides the weak default function.
It calls menu_default_choice() and interprets the data as
struct bootmenu_entry.

pxe boot also uses common menu code for pxe menus.
If there is a system that enables both bootmenu and pxe,
menu_display_statusline() defined in bootmenu.c will be called
and it will interpret struct pxe_label as struct bootmenu_entry.
This leads to data aborts and pxe menu corruptions.

This patch adds support for client defined statusline function
to resolve the above bug.

Signed-off-by: Thirupathaiah Annapureddy <thiruan@linux.microsoft.com>