efi_loader: console support for color attributes

Shell.efi uses this, and supporting color attributes makes things look
nicer.  Map the EFI fg/bg color attributes to ANSI escape sequences.
Not all colors have a perfect match, but spec just says "Devices
supporting a different number of text colors are required to emulate the
above colors to the best of the device’s capabilities".

Signed-off-by: Rob Clark <robdclark@gmail.com>
Tested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Alexander Graf <agraf@suse.de>
[agraf: s/unsigned/unsigned int/]
Signed-off-by: Alexander Graf <agraf@suse.de>