[bios] Support displaying and hiding cursor

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[efi] Support displaying and hiding cursor

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[readline] Ensure cursor is visible when prompting for input

Signed-off-by: Michael Brown <mcb30@ipxe.org>
Merge remote-tracking branch 'upstream/master'