[efi] Ensure local drives are connected when attempting a SAN boot

[efi] Ensure local drives are connected when attempting a SAN boot

UEFI systems may choose not to connect drivers for local disk drives
when the boot policy is set to attempt a network boot.  This may cause
the "sanboot" command to be unable to boot from a local drive, since
the relevant block device and filesystem drivers may not have been
connected.

Fix by ensuring that all available drivers are connected before
attempting to boot from an EFI block device.

Reported-by: Andrew Cottrell <andrew.cottrell@xtxmarkets.com>
Tested-by: Andrew Cottrell <andrew.cottrell@xtxmarkets.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>