Raspberry_pi: fix VT100 console driver

 * Cursor coordinates are 1-based, not 0-based
 * Color change was disabled and broken

This implementation of our console over VT100 is generic and should be
moved out of the raspberry-pi specific folder. However, leaving it there
for now as we will have some bigger reorganization a swe add FDT support
here.
u-boot: sync console implementation from raspberry port

 * VT100 is much more common than VT52 which the u-boot port was
previously using (a legacy of the Atari m68k port)
 * Implement serial_getc (again, code is identical to raspberry port...)
so the boot menu can be used over the serial port. The enter key is
recognized, arrows currently aren't.