efi_loader: auto-generate boot option for each blkio device

Current efibootmgr auto-generates the boot option for all
disks and partitions installing EFI_SIMPLE_FILE_SYSTEM_PROTOCOL,
while EDK II reference implementation auto-generates the boot option
for all devices installing  EFI_BLOCK_IO_PROTOCOL with
eliminating logical partitions.

This commit modifies the efibootmgr to get aligned to EDK II.

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