[undi] Retry PXENV_UNDI_INITIALIZE multiple times

[undi] Retry PXENV_UNDI_INITIALIZE multiple times

On at least one PXE stack (Realtek r8169), PXENV_UNDI_INITIALIZE has
been observed to fail intermittently due to a media test failure (PXE
error 0x00000061).  Retrying the call to PXENV_UNDI_INITIALIZE
succeeds, and the NIC is then usable.

It is worth noting that this particular Realtek PXE stack is already
known to be unreliable: for example, it repeatably fails its own
boot-time media test after every warm reboot.

Fix by attempting PXENV_UNDI_INITIALIZE multiple times, with a short
delay between each attempt to allow the link to settle.

Signed-off-by: Michael Brown <mcb30@ipxe.org>