cmd_pxe.c: Pass along 'cmdtp' to do_bootm()/do_bootz()

When we call do_bootm() with a vmlinuz, this would lead to a NULL
pointer dereference, and after talking with Wolfgang the right thing to
do here for now is to make sure that we pass cmdtp to these functions
rather than NULL.

Reported-by: Steven A. Falco <stevenfalco@gmail.com>
Signed-off-by: Tom Rini <trini@ti.com>