bootmenu: update bootmenu_entry structure

This is a preparation for succeeding addition of uefi boot
and distro boot menu entries into bootmenu.
The bootmenu_entry title is updated to u16 string because
uefi use u16 string. This commit also factors out the function
to prepare the entries generated by "bootmenu_x" U-Boot environment
variable.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>