Fixed IAR serial fgets fgetc
Moved IAR serial differences to retarget.cpp
Merge pull request #770 from GustavWi/iar_mbed

Fix IAR serial fgets fgetc

Taken from PR #770:
setbuf(_file, NULL), and std::setvbuf(_file,NULL,_IONBF,NULL) should both give an unbuffered stream (the data is directly written to the input buffer). IAR sets a buffer anyway of size 512 bytes for these calls. Calling setvbuff(_file,buf,_IONBF,NULL) with a buffer that is not a NULL pointer sets the buffer to size one. Which means that as soon as a char is read it is written to the real buffer. If people are interested in looking at this further they can look at the files under ARM/src/dlib: fgets.c, fflush.c, xfrpep.c and xfwprep.c