cbfs/payload type: Fix build warning and whitespace in name

Currently, adding a payload to CBFS using the build system, the warning
below is shown.

    W: Unknown type 'payload' ignored

Update payload type from "simple elf" to "simple_elf" and rename the
word "payload" to "simple_elf" in all Makefiles.

Fixes: 4f5bed52 (cbfs: Rename CBFS_TYPE_PAYLOAD to CBFS_TYPE_SELF)
Change-Id: Iccf6cc889b7ddd0c6ae04bda194fe5f9c00e495d
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/26240
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
5 files changed