efi_loader: add option to initialise EFI subsystem early

If this option, CONFIG_EFI_SETUP_EARLY, is enabled, the initialisation
of UEFI subsystem will be done as part of U-Boot initialisation.

Please note that this option won't be enabled explicitly by users,
instead, should be enabled implicitly by other configuration options.

Specifically, this feature will be utilised in implementing capsule-on-disk
feature.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>