bootmenu: add UEFI boot entry into bootmenu

This commit adds the UEFI related menu entries
into the bootmenu.

User can select which UEFI "Boot####" option to execute
from bootmenu, then bootmenu sets the "BootNext" UEFI
variable and invoke efi bootmgr. The efi bootmgr
will handle the "BootNext" UEFI variable.

If the "BootNext" UEFI variable is preset and efi bootmgr is enabled,
bootmenu invokes efi bootmgr to handle "BootNext" as first priority.

The UEFI boot entry has the "UEFI BOOTXXXX" prefix as below.

  *** U-Boot Boot Menu ***

     UEFI BOOT0000 : debian
     UEFI BOOT0001 : ubuntu

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>