Add CONFIG_{SD|NAND|ONENAND|SPI|QSPI|SATA}_BOOT kconfig entries.

SoCs supports loading U-Boot from different medias to DRAM, such as
i.MX6/7 supports loading U-Boot to DRAM from sd/emmc/nand/qspi/spi/sata
and etc. For i.MX, imximage will generate different IVT headers according
to boot medias.
Signed-off-by: NPeng Fan <peng.fan@nxp.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Heiko Schocher <hs@denx.de>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Christophe Ricard <christophe-h.ricard@st.com>
Cc: Nikita Kiryanov <nikita@compulab.co.il>
Cc: Francois Retief <fgretief@spaceteq.co.za>
Cc: Tom Rini <trini@konsulko.com>