efi_loader: Add distro boot script for removable media

UEFI defines a simple boot protocol for removable media. There we should look
at the EFI (first GPT FAT) partition and search for /efi/boot/bootXXX.efi with
XXX being different between different platforms (x86, x64, arm, aa64, ...).

This patch implements a simple version of that protocol for the default distro
boot script. With this we can automatically boot from valid UEFI enabled
removable media.

Because from all I could see U-Boot by default doesn't deliver device tree
blobs with its firmware, we also need to load the dtb from somewhere. Traverse
the same EFI partition for an fdt file that fits our current board so that
an OS receives a valid device tree when booted automatically.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>