pxe: Refactor sysboot to have one helper

The only difference between the three helpers is the filesystem type.
Factor this out and call the filesystem functions directly, instead of
through the command-line interpreter. This allows the file size to be
obtained directly, instead of via an environment variable.

We cannot do the same thing with PXE's tftpboot since there is no API
at present to obtain information about the file that was read. So there
is no point in changing pxe_getfile_func to use a ulong for the address,
for example.

This is as far as the refactoring can go for the present.

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>