configs: e850-96: Enable Standard Boot

configs: e850-96: Enable Standard Boot

Enable Standard Boot on E850-96 as documented in [1]. Along with
corresponding changes in the default environment and properly prepared
eMMC partitions (ESP and rootfs), it makes it possible to boot Debian
rootfs automatically. All boot methods were tested: efi_mgr, efi,
syslinux and script. The preferred boot method is efi_mgr, which relies
on the configured EFI variables (stored in /ubootefi.var file on ESP
partition), which boots either GRUB from /EFI/debian/grubaa64.efi, or
systemd-boot from /EFI/BOOT/BOOTAA64.EFI. Currently used boot sequence:

  U-Boot -> bootcmd -> 'bootflow scan -lb' -> efi_mgr -> GRUB -> Debian

[1] doc/develop/bootstd/overview.rst

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>