kbuild: tools: fix a bug that always builds Denx logo

LOGO_BMP was never overwritten by board-specific or
vendor-specific logos.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Reported-by: Bo Shen <voice.shen@atmel.com>
Tested-by: Bo Shen <voice.shen@atmel.com>