dev: Using Configurable image writer in HDLcd

dev: Using Configurable image writer in HDLcd

The fixed image writer (which was dumping .bmp images only) has been
replaced by the configurable one in HDLcd device.  Default format is
Auto, which gives gem5 the freedom to choose the format it prefers.

Change-Id: I0643266556bb10b43cdebd628f6daa2cd5e105dd
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5183
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>