lib/charset: Map Unicode code points to CP437 code points 1-31

lib/charset: Map Unicode code points to CP437 code points 1-31

Code page 437 uses code points 1-31 for glyphs instead of control
characters. Map the appropriate Unicode code points to this code points.
Fixes rendering of grub2's menu as EFI application using the
EFI_SIMPLE_TEXT_OUTPUT_PROTOCOL on a console with bitmap fonts.

Signed-off-by: Janne Grunau <j@jannau.net>