Added a magic character for sd buffering.
if a '#' is read now the buffer will be emptied before reading ahead.
This is so one can execute files from within gcode files, without messing the buffer with preread characters from the caller file.
# can not occure in sd files imho, because it should only occure within checksums in ther serial communication.
Yes, thats a lame argument. If you have a better idea please tell me. It has to be a character that one can type
on a keyboard manually.
Sub-file calls.
by overloading M32 it is now possible to execute gcode files from other gcode files, with a fixed recursion level.
This can be used e.g. for having a real start.g and end.g somewhere on the sd card, which are then called from the normal print file.
Another usecase would be to have macro-files for nozzle-change and layerchange.
I have not tested the speedwise performance. The testing was done with pronterface.

syntax:
normal call from sd card will open the new file and continue executing there.
M32 !/path/filename#
this however will call the new file and return to the caller file.
M32 P !/path/filename#
with the optional "S<position>" the  file starting position can be set.
this is for continuing prints from a previous location.
preparation for hibernation

If a print is stopped, it would be nice in the future to write a file with the printer state, the filename of the print, and the position within the print.
this file could be read, to continue a previously stopped print.
not finished yet.
Merge branch 'Marlin_v1' of https://github.com/ErikZalm/Marlin into Marlin_v1