pxe: Use a context pointer

At present the PXE functions pass around a pointer to command-table entry
which is very strange. It is only needed in a few places and it is odd to
pass around a data structure from another module in this way.

For bootmethod we will need to provide some context information when
reading files.

Create a PXE context struct to hold the command-table-entry pointer and
pass that around instead. We can then add more things to the context as
needed.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Artem Lapkin <email2tema@gmail.com>
Tested-by: Artem Lapkin <email2tema@gmail.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>