imx: Rename CONFIG_MXS to CFG_MXS

This is not a Kconfig option so we should not be setting it in the
Makefile. Rename it to use a CFS_ prefix, since this is still used in
mxsimage.c

In general tools should support all the features without reference to
CONFIG options, but this is left to the maintainer to look at.

Signed-off-by: Simon Glass <sjg@chromium.org>