fb: introduce flush for virtual framebuffer ...

Some drivers need an explicit sync method to flush the virtual
framebuffer to the display. It is called fb_flush().

fb_flush() gets called on fbc_putc, on fb_close and in the pattern cycle
in the fbtest command.

Signed-off-by: Bastian Stender <bst@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

commands: exit on invalid option ...

Barebox commands should not perform any action and return 0 if an
invalid parameter was given. This might cause undetected unintended
behvaior when calling commands with wrong options, either manually or
from a script.

Signed-off-by: Enrico Jorns <ejo@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

GUI: Add fbtest command ...

Add 'fbtest' - a command to produce test patterns on a screen

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Andrey Gusakov <andrey.gusakov@cogentembedded.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>