netboot: improve boot config parsing.
netboot: improve boot config parsing.

...to only parse one boot type for the boot config
url as is. e.g. If it successfully parsed as ipxe
it will less likely (not feasible) to parse as a
simple format.

Signed-off-by: David Hu <a59597aa737a9c4d32eaf7b77687719fab10c4c5@google.com>

u-root/u-root,u-root/u-root,u-root/u-root