bootmenu: use utf-8 for menu title

The commit a3d0aa87 ("bootmenu: update bootmenu_entry structure")
changes the bootmenu title type from char to u16(UTF16 string)
to support EFI based system. If EFI_LOADER is not enabled,
printf("%ls") is not supported, so bootmenu does not appear
correctly.

This commit changes the type of menu title from u16(UTF16) to
utf-8 string and EFI strings is conveted into utf-8.

Fixes: a3d0aa87 ("bootmenu: update bootmenu_entry structure")
Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Tested-by: Pali Rohar <pali@kernel.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>