Use the getopt function in compliance with the standard.

This is a simple change to the use of getopt in reboot.c, as per the standard,
getopt does NOT return an EOF, it returns a -1, (this was done to remove
getopt's dependence on stdio.h), and since the standard does not guarantee 
that EOF is -1, and the documentation tells us that getopt returns -1, 
in my judgement it would be best to test for -1, rather than EOF.

Change-Id: I63a3bb7011eb60753eb910221bbe8ce0a71e6679
Signed-off-by: David Gumberg <davidzgumberg@gmail.com>