efi_loader: add a README.iscsi describing booting via iSCSI

The appended README explains how U-Boot and iPXE can be used
to boot a diskless system from an iSCSI SAN.

The maintainer for README.efi and README.iscsi is set.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
[agraf: s/Adress/Address/]
Signed-off-by: Alexander Graf <agraf@suse.de>