Several UEFI platforms are known to return EFI_NOT_FOUND when asked to
retrieve the system default font information via GetFontInfo().  Work
around these broken platforms by iterating over the glyphs to find the
maximum height used by a printable character.

Originally-fixed-by: Jonathan Dieter <jdieter@lesbg.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>