pxe: Clean up the use of bootfile

The 'bootfile' environment variable is read in the bowels of pxe_util to
provide a directory to which all loaded files are relative.

This is not obvious from the API to PXE and it is strange to make the
caller set an environment variable rather than pass this as a parameter.

The code is also convoluted, which this feature implemented by
get_bootfile_path().

Update the API to improve this. Unfortunately this means that
pxe_setup_ctx() can fail, so add error checking.

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>