build: Use generic boot logo matching

The boot logo matching is now done in following way:

 - use LOGO_BMP if it is set, or
 - use $(BOARD).bmp if it exists in tools/logos, or
 - use $(VENDOR).bmp if it exists in tools/logos, or
 - use denx.bmp otherwise.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Acked-by: Wolfgang Denk <wd@denx.de>