efi_loader: re-enable GRUB workaround on 32bit ARM

GRUB on ARM 32bit prior to version 2.04 lacks proper handling of caches.
In U-Boot v2019.04 a workaround for this was inadvertently removed.

The workaround is currently also needed for booting on systems with caches
that cannot be managed via CP15 (e.g. with an i.MX6 CPU).

Re-enable the workaround and make it customizable.

Fixes: f69d63fa ("efi_loader: use efi_start_image() for bootefi")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>