Currently the U-Boot doc/ directory contains the following files
that are only relevant for i.MX devices:

- doc/README.imx25
- doc/README.imx27
- doc/README.imx5
- doc/README.imx6
- doc/README.imximage
- doc/README.mxc_hab
- doc/README.mxs
- doc/README.mxsimage
- doc/README.sdp

Move all content to a common i.MX folder for a better documentation
structure.

Signed-off-by: Breno Lima <breno.lima@nxp.com>

The changes required to use U-Boot's Serial Download Protocol
implementation are now available in upstream imx_usb_loader
repository. Update the URL accordingly.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Reviewed-by: Tom Rini <trini@konsulko.com>

Document the U-Boot Serial Download Protocol implementation and
some typical use cases.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Reviewed-by: Łukasz Majewski <lukma@denx.de>