OMAP3: overo : Add environment variable optargs to bootargs

This allows the user can easily add extra kernel arguments. Very helpful
for reserving memory for the DSP without rewriting the entire kernel
argument line.

Signed-off-by: Philip Balister <philip@opensdr.com>
Tested-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>