boot: correct finding the default EFI binary

boot: correct finding the default EFI binary

* The sandbox must not use an arbitrary file name bootsbox.efi but the
  file name matching the host architecture to properly boot the respective
  file. We already have an include which provides a macro with the name of
  the EFI binary. Use it.

* The path to the EFI binary should be absolute.

* The path and the file name must be capitalized to conform to the UEFI
  specification. This is important when reading from case sensitive
  file systems.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Tested-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>