Add a centralised concept of colours and colour pairs (using the
default colour pairs as configured via config/colour.h).  A colour
pair consists of a pair of colour indices.

Add the ability to redefine both a colour pair and an individual
colour index, with minimal overhead if this feature is not required
(e.g. because the relevant shell commands are not present in the
build).

Signed-off-by: Michael Brown <mcb30@ipxe.org>