fbsplash: support configurable image position

fbsplash: support configurable image position

For some setups (E.G. for supporting different screen resolutions),
positioning the image somewhere else than the top left corner may be
interesting.

Add support for IMG_LEFT/IMG_TOP settings to specify the image location,
similar to how it is done for the progress bar.

function                                             old     new   delta
fbsplash_main                                        994    1038     +44
static.param_names                                    57      74     +17
packed_usage                                       32631   32647     +16
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(add/remove: 0/0 grow/shrink: 3/0 up/down: 77/0)               Total: 77 bytes

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>