kern/ieee1275/init: Use net config for boot location instead of firmware bootpath

kern/ieee1275/init: Use net config for boot location instead of firmware bootpath

On network boots grub_ieee1275_net_config() is used to determine the
boot device but the path continues to be taken from the Open Firmware
/chosen/bootpath property. This assumes the device node follows the
generic IEEE 1275 syntax which is not always the case. Different drivers
may extend or redefine the format and GRUB may then misinterpret the
argument as a filename and set $prefix incorrectly.

The generic Open Firmware device path format is:

  device-name[:device-argument]
  device-argument := [partition][,[filename]]

For example, a bootpath such as:

  /vdevice/l-lan@30000002:speed=auto,duplex=auto,1.2.243.345,,9.8.76.543,1.2.34.5,5,5,255.255.255.0,512

does not follow this form. The section after the colon (the device-argument)
contains driver-specific options and network parameters, not a valid filename.
The GRUB interprets this string as a filename which results in $prefix being
set to "/", effectively losing the intended boot directory.

The firmware is not at fault here since interpretation of device nodes
is driver-specific. Instead, GRUB should use the filename provided in
the cached DHCP packet which is consistent and reliable. This is also
the same mechanism already used on UEFI and legacy BIOS platforms.

This patch updates grub_machine_get_bootlocation() to prefer the result
from grub_ieee1275_net_config() when complete and only fall back to the
firmware bootpath otherwise.

Signed-off-by: Michael Chang <mchang@suse.com>
Reviewed-by: Avnish Chouhan <avnish@linux.ibm.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>