uboot-imx

README: Drop information about commands

Most of this is duplicated in Kconfig help. Add some of that which is not,
and remove the help from the README.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>