Config: Add command line options for disk image and memory size

Config: Add command line options for disk image and memory size
Added the options to Options.py for FS mode with backward compatibility. It is
good to provide an option to specify the disk image and the memory size from
command line since a lot of disk images are created to support different
benchmark suites as well as per user needs. Change in program also leads to
change in memory requirements. These options provide the interface to provide
both disk image and memory size from the command line and gives more
flexibility.