efi: add a command to read efi variables from shell and configuration files

[Ignat Korchagin]

Would like to add a command for reading EFI variables from command line or config files to make following scenario possible in grub.cfg:

# check whether Secure Boot is enabled
get_efivar -t uint8 SecureBoot efi_secure_boot

if [ $efi_secure_boot = 1 ]; then
# secure boot is enabled, boot our secure OS
elif
# secure boot is disabled: enable it, boot insecure OS etc
fi 

grub2.patch
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