MFC r337131, r337231: stand: efipart fixes

r337131:
loader.efi: efipart_inithandles() failure should not prevent devsw initialization

The efipart_inithandles() can return error code when there are no block devices,
however, we still should walk and initialize devsw.

r337231:
loader.efi: clean up misleading noise from missing block devices

If there are no block devices, there is no need to printout
error (ENOENT).

In case of netboot, our image path has no block device, no need to make
noise about it.