'read' function should return 0 when encountering an end of file. When
newlib calls read and sees EOF returned, it assumes that this
condition is permanent and never calls 'read' for this file again
(unless the read pointer is moved using fseek).
The correct behavior in case no characters were received over UART is
to return -1. In this case newlib will retry reading from file on next
call to fread, fgetc or another function which calls __srefill_r.
77b754b47f