efi_loader: bootmgr: support BootNext and BootCurrent variable behavior

See UEFI v2.7, section 3.1.2 for details of the specification.

With efidebug command, you can run any EFI boot option as follows:
  => efi boot add 1 SHELL ...
  => efi boot add 2 HELLO ...
  => efi boot order 1 2
  => efi bootmgr
     (starting SHELL ...)

  => efi boot next 2
  => efi bootmgr
     (starting HELLO ...)
  => env print -e
  <snip ...>
  BootCurrent: {boot,run}(blob)
  00000000:  02 00                    ..
  BootOrder: {boot,run}(blob)
  00000000:  01 00 02 00              ....

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>