cmd_bootm: Add command line arguments to Plan 9

cmd_bootm: Add command line arguments to Plan 9

This patch introduces support for command line arguments to Plan 9.
Plan 9 generally dedicates a small region of kernel memory (known
as CONFADDR) for runtime configuration.  A new environment variable
named confaddr was introduced to indicate this location when copying
arguments.

Signed-off-by: Steven Stallion <sstallion@gmail.com>
[trini: Adapt for Simon's changes about correcting argc, no need to bump
by 2 now]
Signed-off-by: Tom Rini <trini@ti.com>