efi_loader: split README.efi into two separate documents

README.efi describes two different concepts:
* U-Boot exposing the UEFI API
* U-Boot running on top of UEFI.

This patch splits the document in two.
Religious references are removed.

The separation of the concepts makes sense before detailing the internals
of U-Boot exposing the UEFI API in a future patch.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>