tools: mkenvimage: Fix possible segfault on stdin input

tools: mkenvimage: Fix possible segfault on stdin input

The size of 'filebuf' was not increased as more and more bytes are read
from stdin, but 'filebuf' was always reallocated to the same fix size.
This works as long as only less bytes than the initial buffer size come
in, for more input this will segfault. (It actually does, I tested
that.) So for each loop cycle the buffer size has to be increased by the
number of bytes we want to read.

Signed-off-by: Alexander Dahl <ada@thorsis.com>