Add instructions to enable RTC in the nsh/ configuration; RTC enabled by default in the demo configuration

SAMA5 LCD:  Move framebuffers to center of free memory region.  That creates a guard band around the framebuffers that gives a little protection from any bad writes into the framebuffer

Minor tweaks to debug output; started renaming some icons to include size in the name; SAMA5 NxWM config needs libm

New touchscreen scaling algorithm for the case where measured X values vary with y position (and vice versa)