[process] Pass containing object pointer to process step() methods

[process] Pass containing object pointer to process step() methods

Give the step() method a pointer to the containing object, rather than
a pointer to the process.  This is consistent with the operation of
interface methods, and allows a single function to serve as both an
interface method and a process step() method.

Signed-off-by: Michael Brown <mcb30@ipxe.org>