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 <[e-mail]>
Signed-off-by: Minkyu Kang <[e-mail]>

configs/e850-96_defconfig

···
 # CONFIG_PSCI_RESET is not set
 CONFIG_EFI_SET_TIME=y
 CONFIG_ANDROID_BOOT_IMAGE=y
-# CONFIG_AUTOBOOT is not set
+CONFIG_BOOTSTD_FULL=y
 CONFIG_DEFAULT_FDT_FILE="exynos850-e850-96.dtb"
 # CONFIG_DISPLAY_CPUINFO is not set
-CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTEFI_SELFTEST=y
 CONFIG_CMD_ABOOTIMG=y
 CONFIG_CMD_NVEDIT_EFI=y
 CONFIG_CMD_CLK=y
 CONFIG_CMD_GPT=y
 CONFIG_CMD_MMC=y
-CONFIG_CMD_PART=y
 CONFIG_CMD_EFIDEBUG=y
 # CONFIG_CMD_DATE is not set
 CONFIG_CMD_RTC=y