linux: add custom linux boot logo option

This patch adds a simple way to change the linux bootup logo.
The patch was kept purposely simple to support only the use cause
where a user needs a color linux boot up logo.

Signed-off-by: Angelo Compagnucci <angelo@amarulasolutions.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[Peter: clarify/reword option/help text, automatically enable framebuffer
	support, fix convert path]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>