pxe: Move API comments to the header files

Put the function comments in the header file so that the full API can we
examined in one place.

Expand the comments to cover parameters and return values.

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>
2 files changed